Abstract:
In an Internet telephone system for implementing bilateral communication over the Internet between telephone sets and communication devices, by causing the telephone sets and the communication devices to connect to a repeater device via a public network so as to establish a communication channel on the Internet via the repeater device, each of the communication devices comprises: a connection unit for dialing one of a remote telephone set and a remote repeater device via the public network when a call is originated within the communication device or by a local telephone set connected to the communication device, so as to establish a connection; and a communication unit for performing data communication over the public network and over the Internet, wherein the repeater device is controlled by the connection unit and the communication unit to establish the communication channel on the Internet so that users of the local telephone set and the remote telephone set may perform speech communication over the Internet, using the same procedure as employed when the users perform speech communication over the public network.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to the Internet and, more particularly, to an Internet telephone system which uses the Internet for speech communication between telephone sets and computers. 
     2. Description of the Related Art 
     The Internet telephone system according to the related art using the Internet for communication between telephones and computers is implemented by a construction shown in FIG.  1  and FIG.  2 . 
     FIG. 1 shows an Internet telephone system for enabling speech communication between computers. 
     Referring to FIG. 1, the Internet telephone system comprises a computer  101 , a computer  102 , a dedicated server  103 , and an Internet  105 . The dedicated server  103  has a management database (DB)  104  that centrally manages IP addresses of all the computers connected to the Internet  105 . 
     For example, when the computer  101  connects to the computer  102 , the computer  101  connects to the dedicated server  103  first, and confirms whether an IP address of the target computer  102  is registered in the management database  104 . For example, the IP address information referred to in the confirmation may be a name with which the computer  102  is managed on a network. 
     The dedicated server  103  notifies the computer  101  of the IP address if it is registered in the management database  104 , so that the computer  101  uses the IP address to connect to the computer  102  and communicate over the Internet  105 . Each of the computers  101  and  102  should be provided with a microphone for speech communication with each other. 
     FIG. 2 shows an Internet telephone system which uses computers to enable telephone sets to perform speech communication. 
     Referring to FIG. 2, the Internet telephone system comprises a telephone set  111 , a telephone set  112 , a computer  113 , a computer  114 , and an Internet  117 . The computer  113  and the computer  114  are provided with a management database  115  and a management database  116 , respectively, to manage IP addresses of computers connected to Internet  117  and phone numbers of the telephone sets connected to the respective computers. 
     For example, the telephone set  111  connects to the nearest computer  113  in order to call the telephone set  112 . The computer  113  executes a user authorization process for determining whether the telephone set  111  is qualified to use the Internet  117 . 
     The telephone set  111  dials a phone number of the telephone set  112  when it is determined that the telephone set  111  is qualified to use the Internet. The computer  113  then connects to the computer  114  nearest the telephone set  112 . The computer  114  connects to the telephone set  112  so as to enable speech communication over the Internet  117  between the telephone set  111  and the telephone set  112 . 
     However, the Internet telephone system according to the related art is inconvenient in that it imposes a more complicated operation on a call originator than when an ordinary telephone set is used. More specifically, the call originator is required to register personal information in a managing computer located on a network. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an Internet telephone system in which the aforementioned problems are eliminated. 
     Another and more specific object of the present invention is to provide an Internet telephone system whereby a user can operate a telephone set as any ordinary telephone set. 
     Still another object of the present invention is to provide an Internet telephone system which realizes low-cost speech communication using the Internet. 
     The aforementioned objects can be achieved by an Internet telephone system for implementing bilateral communication over the Internet between telephone sets and communication devices, by causing the telephone sets and the communication devices to connect to a repeater device via a public network so as to establish a communication channel on the Internet via the repeater device, wherein each of the communication devices comprises: connection means for dialing one of a remote telephone set and a remote repeater device via the public network when a call is originated within the communication device or by a local telephone set connected to the communication device, so as to establish a connection; and communication means for performing data communication over the public network and over the Internet, wherein the repeater device is controlled by the connection means and the communication means to establish the communication channel on the Internet so that users of the local telephone set and the remote telephone set may perform speech communication over the Internet, using the same procedure as employed when the users perform speech communication over the public network. 
     Each of the communication devices may further comprise: registering means for registering predetermined control information transmitted to and received from a remote communication device connected to the remote telephone set; and information storage means for storing the control information registered by the registering means, wherein the connection means and the communication means may control the repeater device in accordance with the control information. 
     Each of the communication devices may further comprise: monitoring means for monitoring communication data on the Internet while an Internet call using the Internet is proceeding; and switching means for switching the Internet call using the Internet to a public circuit call using the public network, when it is determined that a predetermined transfer rate for the communication data fails to be guaranteed. 
     Each of the communication devices may comprise activating means for automatically activating hardware and software necessary for a call, when it is determined upon origination of a call by a telephone set on an originating side that a main power supply of the communication device is on but the necessary hardware and the software are not activated. 
     Each of the communication devices may comprise converting means for converting analog data output from a telephone set on an originating side into digital data, and converting digital data into analog data to be supplied to a destination telephone set. 
     The repeater device may comprise conversation data storage means for storing received conversation data when the received conversation data cannot be transmitted to the communication device. 
     The aforementioned objects can also be achieved by a communication device for an Internet telephone system for implementing bilateral communication over the Internet, by connecting to a telephone set and connecting to a repeater device via a public network, the communication device comprising: connection means for dialing one of a remote telephone set and a remote repeater device via the public network when a call is originated by a local telephone set connected to the communication device, so as to establish a connection; and communication means for performing data communication over the public network and over the Internet, wherein the repeater device is controlled by the connection means and the communication means to establish a communication channel on the Internet so that users of the local telephone set and the remote telephone set may perform speech communication over the Internet, using the same procedure as employed when the users perform speech communication over the public network. 
     The communication device may further comprise: registering means for registering predetermined control information transmitted to and received from a remote communication device connected to the remote telephone set; and information storage means for storing the control information registered by the registering means, wherein the connection means and the communication means may control the repeater device in accordance with the control information. 
     The communication device may further comprise: monitoring means for monitoring communication data on the Internet while an Internet call using the Internet is proceeding; and switching means for switching the Internet call using the Internet to a public circuit call using the public network, when it is determined that a predetermined transfer rate for the communication data fails to be guaranteed. 
     The communication device may further comprise: activating means for automatically activating hardware and software necessary for a call, when it is determined upon origination of a call by a telephone set on an originating side that a main power supply of the communication device is on but the necessary hardware and the software are not activated. 
     The communication device may further comprise converting means for converting analog data outputted from a telephone set on an originating side into digital data, and converting digital data into analog data to be supplied to a destination telephone set. 
     The aforementioned objects can also be achieved by a repeater device for an Internet telephone system for implementing bilateral communication over the Internet, by connecting to telephone sets and communication devices via a public network, comprising conversation data storage means for storing received conversation data when the received conversation data cannot be transmitted to a communication device. 
     The aforementioned objects can also be achieved by a machine-readable recording medium storing a program for implementing bilateral communication over the Internet between telephone sets and communication devices, by causing the telephone sets and the communication devices to connect to a repeater device via a public network so as to establish a communication channel on the Internet via the repeater device, the program comprising the steps of: causing a communication device to dial one of a remote telephone set and a remote repeater device via the public network when a call is originated by one of a local telephone set and the communication device, so as to establish a connection; and causing the communication device to perform data communication over the public network and over the Internet, the repeater device being controlled by the steps to establish the communication channel on the Internet so that users of the local telephone set and the remote telephone set may perform speech communication over the Internet, using the same procedure as employed when the users perform speech communication over the public network. 
     The program may further comprise the steps of: causing the communication device to register predetermined control information transmitted to and received from a remote communication device connected to the remote telephone set; and storing the control information registered in the above step. 
     The program may further comprise the steps of: monitoring communication data on the Internet while an Internet call using the Internet is proceeding; and switching the Internet call using the Internet to a public circuit call using the public network, when it is determined that a predetermined transfer rate for the communication data fails to be guaranteed. 
     The program may further comprise the steps of automatically activating hardware and software necessary for a call, when it is determined upon origination of a call by a telephone set on an originating side that a main power supply of the communication device is on but the necessary hardware and the software are not activated. 
     The program further may comprise the steps of converting analog data output from a telephone set on an originating side into digital data, and converting digital data into analog data to be supplied to a destination telephone set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows an Internet telephone system according to the related art; 
     FIG. 2 shows another Internet telephone system according to the related art; 
     FIG. 3 shows a construction of a communication device according to the present invention; 
     FIG. 4 shows a principle and construction of an Internet telephone system according to the present invention; 
     FIG. 5 shows a constitution and operation of the Internet telephone system according to the present invention; 
     FIG. 6 shows a construction of the Internet telephone system; 
     FIG. 7 shows an operation whereby information is exchanged between computers; 
     FIG. 8 shows an example of information managed by an information storage unit; 
     FIG. 9 shows an example of information exchanged between computers; 
     FIG. 10 shows control executed between computers; 
     FIG. 11 shows control executed between computers; 
     FIG. 12 shows an operation performed between an originating computer and a service provider; 
     FIG. 13 shows an example of information stored by an information storage unit; 
     FIG. 14 shows an operation performed between service providers; 
     FIG. 15 shows an operation performed between a service provider on the receiving side and a computer; 
     FIG. 16 shows an automatic switch between the Internet and a public network; 
     FIG. 17 shows an operation performed between the Internet telephone system and an existing telephone set; 
     FIG. 18 shows control executed between the Internet telephone system and an existing telephone set; 
     FIG. 19 shows an operation performed between the Internet telephone system and an existing telephone set; 
     FIG. 20 shows control executed between the Internet telephone system and an existing telephone set; 
     FIG. 21 shows an answer phone operation; 
     FIG. 22 shows analog/digital conversion; 
     FIG. 23 shows an operation performed between computers; 
     FIG. 24 shows an operation performed between a service provider on the receiving side and a computer; 
     FIG. 25 is a flowchart of a process performed in an IP Internet telephone system; 
     FIG. 26 is a flowchart of a process performed in a dial-up Internet telephone system; 
     FIG. 27 is a flowchart of a process performed in a dial-up Internet telephone system; 
     FIG. 28 is a flowchart of a process performed between a dial-up Internet telephone system and an existing telephone set; 
     FIG. 29 is a flowchart of a process performed on the originating side of an IP Internet telephone system; 
     FIG. 30 is a flowchart of a process performed on the receiving side of an IP Internet telephone system; 
     FIG. 31 is a flowchart of a process performed on the originating side of a dial-up Internet telephone system; 
     FIG. 32 is a flowchart of a process performed on the receiving side of a dial-up Internet telephone system; 
     FIG. 33 is a flowchart of another process performed on the originating side of a dial-up Internet telephone system; and 
     FIG. 34 is a flowchart of a process performed on the originating side of a dial-up Internet telephone system when communication proceeds between the dial-up Internet telephone system and an existing telephone set. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 4 shows a principle and construction of the Internet telephone system of the present invention. 
     Referring to FIG. 4, the Internet telephone system comprises a first call device  1 , a second call device  2 , a first communication device  3 , a second communication device  4 , a first repeater device  6  and a second repeater device  7 . Each of the first call device  1  and the second call device  2  is implemented by a telephone set. The first communication device  3  is implemented by a computer which performs necessary connection and communication processes so that the first call device  1  can perform bilateral communication over a public network  5 . The second communication device  4  is implemented by a computer which performs necessary connection and communication processes so that the second call device  2  can perform bilateral communication over the public network  5 . Each of the first repeater device  6  and the second repeater device  7  is implemented by a service provider which uses the public network  5  as a connection medium for connecting to communication devices and performs necessary connection and communication processes for bilateral communication over an Internet  8 . 
     For example, when a phone number for the second call device  2  is input by a user of the first call device  1 , the first communication device  3  transmits and receives information that enables communicating over the Internet  8 , to and from the second communication device  4  via the public network  5 . 
     The first communication device  3  and the second communication device  4 , which transmit and receive the information, connect to the first repeater device  6  and the second repeater device  7 , respectively, in accordance with the information. The first repeater device  6  and the second repeater device  7  are connected to each other over the Internet  8 . In this state, conversation data from the first call device  1  and the second call device  2  can travel over the Internet. 
     The first communication device  3  and the second communication device  4  have facilities to automatically switch from the communication using the Internet  8  to the communication using the public network  5  when it becomes impossible to guarantee a predetermined transmission rate on the Internet  8 . 
     FIG. 5 shows a construction and an operation of the Internet telephone system which uses the principle and the construction shown in FIG.  4 . 
     Referring to FIG. 5, the Internet telephone system comprises a telephone set  11 , a telephone set  12 , a telephone set  13 , a computer  14 , a computer  15 , a service provider  17 , and a service provider  18 . The computer  14  performs necessary connection and communication processes so that the telephone set  11  can perform bilateral communication over a public network  16 . The computer  15  performs necessary connection and communication processes so that the telephone set  12  can perform bilateral communication over the public network  16 . The service provider  17  and the service provider  18  use the public network  16  as a connection medium for connecting to computers, and perform necessary connection and communication processes to enable bilateral communication over an Internet  19 . 
     When a user of the telephone set  11  performs a dialing process to call the telephone set  12  in the Internet telephone system constructed as above, the computer  14  connected to the telephone set  11  communicates with the computer  15  connected to the telephone set  12 , so as to transmit and receive respective phone numbers and IP addresses over the public network  16  (S 1 ). 
     The computer  14  connects to the nearest service provider  17  over the public network  16 . The service provider  17  receives an IP address of a call destination and connects to the service provider  18  over the Internet  19 . The service provider  18  connects to the computer  15  connected to the telephone set  12 , over the public network  16 . In this state, the telephone set  11  and the telephone set  12  can perform speech communication as in any ordinary telephone call (S 2 ). Release of the network  16  and the Internet  19  after the conversation is performed in one of the telephone sets which is the first to be placed in an on-hook state. 
     When a dialing process is performed by a user of the telephone set  11  to call the telephone set  13  to which no computer is connected, the computer  14  connected to the telephone set  11  communicates with the telephone set  13  so as to transmit and receive respective phone numbers and IP addresses over the public network  16  but fails because the telephone set  13  is not connected to a computer. 
     With this failure, the computer  14  recognizes that a computer is not connected to the telephone set  13 , and that it is possible to perform speech communication with the telephone set  13  over the public network  16 . Therefore, conversation data from the telephone set  11  and the telephone set  13  is transmitted and received via the computer  14 , so that the telephone set  11  and the telephone set  13  can perform speech communication as in any ordinary telephone call (S 3 ). 
     It is to be noted that conversation over the public network  16  is possible as in any ordinary telephone call when a telephone set on the originating side is not connected to a computer. 
     A detailed construction of the Internet telephone system shown in FIG.  5  and the control of signals therein will be explained with reference to FIGS. 6-24. 
     FIG. 6 shows a detailed construction of the Internet telephone system of the present invention. 
     Referring to FIG. 6, the Internet telephone system constructed as shown in FIG. 5 comprises the telephone set  11 , the computer  14 , the public network  16 , the service provider  17  and the Internet  19 . The computer  14  comprises a connection unit  31 , a disconnection unit  32 , a conversion unit  33 , a communication unit  34 , a registering unit  35 , a monitoring unit  36 , a switching unit  37 , a power-on unit  38 , and an information storage unit  39 . The computer  14  communicates with other computers and service providers over the public network  16 . The detailed construction of the computer  15  to be described later is the same as the construction of the computer  14 . 
     The service provider  17  comprises a connection unit  51 , a communication unit  52 , a disconnection unit  53 , a registering unit  54 , and an information storage unit  55 . With this construction, transmission and reception of conversation data using the Internet  19  is made possible. The detailed construction of the service provider  18  to be described later is the same as that of the service provider  17 . 
     When the telephone set  11  transmits a phone number in the Internet telephone system constructed as in FIG. 6, the connection unit  31  of the computer  14  connects to a computer connected to a call destination telephone set (hereinafter, a call destination computer) over the public network  16  and also connects to the service provider  17  over the public network  16 . 
     The disconnection unit  32  provides facilities for terminating a connection with the call destination computer, and also for terminating a connection with the service provider  17  connected to the computer  14  via the public network  16 . 
     The conversion unit  33  provides facilities to perform digital/analog conversion of conversation data between the telephone set  11  and the destination computer. 
     The communication unit  34  provides facilities to exchange conversation data and control data with the call destination computer, over the public network  16 . Moreover, the communication unit  34  provides facilities to perform a similar communication with the service provider  17  over the public network  16 . 
     The registering unit  35  provides facilities for receiving information from the call destination computer, over the public network  16 . 
     For example, the registering unit  35  receives a phone number and an IP address and registers the same in the information storage unit  39  to be described later. 
     The monitoring unit  36  provides facilities for monitoring whether the Internet  19  is not loaded to such an extent that a predetermined transmission rate cannot be guaranteed. More specifically, the monitoring unit  36  monitors the traffic so as to determine whether the transmission rate is lowered to such an extent that a call originator and a call recipient engaged in a telephone call feel uncomfortable. 
     The switching unit  37  provides facilities for automatically switching from the communication using the Internet  19  to the communication using the public network  16 , when the predetermined transmission rate of the Internet  19  fails to be guaranteed. 
     The power-on unit  38  provides facilities for automatically turning on a power supply of the computer  14 , when the power supply of the computer  14  is not on when it is called by a call origination telephone set. 
     The information storage unit  39  provides facilities for storing information to control a call destination telephone set. For example, the information storage unit  39  stores a phone number and an IP address. 
     In the Internet telephone system constructed as shown in FIG. 6, the connection unit  51  of the service provider  17  provides facilities for connecting over the Internet  19  to the service provider connected to a call destination computer (hereinafter, a call destination service provider). 
     The disconnection unit  53  provides facilities for terminating a connection with the call destination service provider connected over the Internet  19 . 
     The communication unit  52  provides facilities for transmitting and receiving, over the Internet  19 , conversation data and control data to and from the call destination service provider. The communication unit  52  also provides facilities for transmitting and receiving conversation data and control data to and from the computer  14  over the public network  16 . 
     The registering unit  54  provides facilities for registering conversation data received from a call destination telephone set in the information storage unit  55  to be described later, when the conversation data fails to be transmitted to the computer  14 . 
     The information storage unit  55  provides facilities for storing the conversation data which has failed to be transmitted to the computer  14 . The conversation data thus stored can be reproduced subsequently. 
     The Internet telephone system constructed as shown in FIG. 6 may be constructed such that the computer  14  is built into the telephone set  11 . In this case, the computer  14  is not a prerequisite. 
     For example, the public network  16  may be an analog telephone line, a digital telephone line, a CATV (cable television), or a satellite line. 
     The service provider  17  is indispensable in order to use the Internet  19 . 
     FIG. 7 shows an operation to transmit and receive information between the computer  14  and the computer  15  in the Internet telephone system shown in FIG.  5 . In transmission and reception of information between the computer  14  and the computer  15 , the connection unit  31 , the communication unit  34 , the registering unit  35  and the information storage unit  39  of the computer  14  shown in FIG. 6 operate. As for the computer  15 , a connection unit  31   a , a communication unit  34   a , a registering unit  35   a  and a information storage unit  39   a , respectively having the same facilities as the equivalent units of the computer  14 , operate. 
     When a user of the telephone set  11  dials a phone number of the call destination telephone set connected to the computer  15  (S 11 ), the connection unit  31  of the computer  14  searches the information storage unit  39  to determine whether a phone number and an IP address of the destination are registered therein (S 12 ). 
     When the phone number and the IP address are registered, transmission and reception of information are not performed between the computer  14  and the computer  15 . 
     When the phone number and the IP address are not registered, the connection unit  31  of the computer  14  dials the connection unit  31   a  of the computer  15  over the public network  16  (S 13 ). With this, a communication as in any ordinary telephone call becomes possible when the dialing is successful. Subsequently, the communication unit  34  of the computer  14  requests an IP address from the communication unit  34   a  of the computer  15  (S 14 ). The communication unit  34  simultaneously notifies the communication unit  34   a  of the phone number and the IP address of the telephone set  11  managed by the information storage unit  39 . 
     When the communication unit  34   a  has received the phone number and the IP address, the registering unit  35   a  registers the phone number and the IP address in the information storage unit  39   a  (S 15 ). 
     The communication unit  34   a  reads out the phone number and the IP address of the telephone set  12  registered in the information storage unit  39   a  (S 16 ) and reports the phone number and the IP address thus read out to the communication unit  34  (S 17 ). When the communication unit  34  receives the phone number and the IP address of the telephone set  12 , the registering unit  35  registers the phone number and the IP address in the information storage unit  39  (S 18 ). The information registered in the information storage unit  39  and the information storage unit  39   a  are managed and used as information necessary in the speech communication over the Internet  19 . 
     FIG. 8 shows an example of information managed by the information storage unit  39  and the information storage unit  39   a  of the present invention. 
     For example, the information managed by the information storage unit  39  comprises network information  61  of the telephone set  11 , service provider information  62  and destination network information  63  of a call destination telephone set. Storage in the information storage unit  39  is controlled by the registering unit  35 . The information managed by the information storage unit  39   a  comprises network information of the telephone set  12 , service provider information, and destination network information of the call destination telephone set. Storage in the information storage unit  39   a  is controlled by the registering unit  35   a.    
     The network information  61  of the telephone set  11  is used in order to manage a phone number and an IP address of the telephone set  11 . 
     The service provider information  62  is used in order to manage a phone number, a user ID and a user authentification password to access a service provider. 
     The destination network information  63  is used to manage a phone number and an IP address of a call destination telephone set. The destination network information  63  is also used to determine whether the Internet connection is used or the ordinary public line is used to connect to the call destination telephone set. For example, information ‘1’ and ‘0’ is managed. 
     FIG. 9 shows an example of information transmitted and received over the public network  16  between the computer  14  and the computer  15 . 
     The communication unit  34  of the computer  14  requests an IP address from the communication unit  34   a  and simultaneously reads out a phone number ‘012345678’ and an IP address ‘12345678900’ of the telephone set  11  from the network information  61  registered in the information storage unit  39 , and notifies the communication unit  34   a  of the information thus read out. 
     The communication unit  34   a  which receives the information described above registers the information as destination network information  63   a , reads out a phone number ‘987654321’ and an IP address ‘98765432100’ of the telephone set  12  from network information  61   a  registered in the information storage unit  39   a  and notifies the communication unit  34  of the information. The communication unit  34  which receives the phone number and the IP address from the communication unit  34   a  registers the information in the information storage device  39 . 
     FIG. 10 shows the control performed when an IP Internet connection is established between the computer  14  and the computer  15  in the Internet telephone system shown in FIG.  5 . 
     In this case, each of the computer  14  and the computer  15  has a native IP address. Moreover, the information storage unit  39  stores an IP address of the computer  15  and the information storage unit  39   a  stores an IP address of the computer  14 . In this state, the computer  14  establishes an IP connection to the Internet  19  through the service provider  17 , and the computer  15  establishes an IP connection to the Internet  19  through the service provider  18 . The computers  14  and  15  supply various control data over this communication path. 
     FIG. 11 shows the control performed in the Internet telephone system shown in FIG. 5, in a case where the computer  14  establishes an IP connection to the Internet, and the computer  15  establishes a dial-up connection to the Internet. 
     In this case, the computer  14  has a native IP address, and the computer  15  does not have a native IP address. The service provider  18  to which the computer  15  is connected has an IP address. In this state, the communication unit  34  of the computer  14  connects to the service provider  18  over the Internet  19  through the service provider  17 . A PPP connection that uses the public network is established between the communication unit  34   a  of the computer  15  and a communication unit  52   a  of the service provider  18 . The computer  14  and the computer  15  supply various control data over this communication path. 
     FIG. 12 shows an operation performed in the Internet telephone system shown in FIG. 5 for transmitting and receiving information over the public network  16  between the computer  14  and the service provider  17  on the originating side. In transmission and reception of information between the computer  14  and the service provider  17 , the connection unit  31 , the communication unit  34 , the information storage unit  39  of the computer  14  shown in FIG. 6 operate, and the connection unit  51 , the communication unit  52  and information storage unit  55  of the service provider  17  operate. 
     For example, the connection unit  31  of the computer  14  reads out ‘11-111-1111’ as a service provider phone number registered in the information storage unit  39  (S 21 ), and dials the connection unit  51  of the service provider  17  (S 22 ). 
     The communication unit  52  of the service provider  17  requests a user ID and a password of the computer  14  from the communication unit  34  in order to authenticate a user (the computer  14 ) (S 23 ). In response to the demand for the user ID and the password, the communication unit  34  of the computer  14  reads out a user ID and a password registered in the information storage unit  39 . The communication unit  34  also reads out a destination telephone number and a destination IP address (S 24 ). For example, the communication unit  34  reads out an ID ‘U01’, a password ‘PSW01’, a destination phone number ‘98-765-4321’ and a destination IP address ‘987.654.32.100’ and notifies the communication unit  52  of the service provider  17  accordingly (S 25 ). Simultaneously, the communication unit  34  notifies the communication unit  52  of the telephone number and the IP address of the computer  14 . 
     In this state, the communication unit  52  examines the user ID and the password registered in the information storage unit  55  (S 26 ). When the passwords match and the user IDs match, the communication unit  52  permits an Internet connection. The communication unit  52  refuses a demand of access from a user (the computer  14 ) when the passwords and the user IDs do not match. 
     FIG. 13 shows an example of information managed by the information storage unit  55  referred to when the service provider  17  examines the registered user ID and password. 
     Information managed by the information storage unit  55  comprises provider connection information  71  and answer phone information  72 . Storage of information in the information storage unit  55  is controlled by the registering unit  54  of the service provider  17  shown in FIG.  6 . 
     The provider connection information  71  registered in the information storage unit  55  is used to manage a user phone number, a user ID and a password to authenticate a user of the service provider. For example, a phone number ‘01-234-5678’, a user ID ‘U01’, and a password ‘PSW01’ are managed. 
     In the answer phone information  72 , ‘U01.WAV’, for example, is managed as sound data corresponding to a user phone number ‘01-234-5678’. 
     FIG. 14 shows an operation for transmitting and receiving information between the service provider  17  on the originating side  17  and the service provider  18  on the receiving side in the Internet telephone system shown in FIG.  5 . In transmission and reception of information over the Internet  19  between the service provider  17  and the service provider  18 , the connection unit  51  and the communication unit  52  of the service provider  17  operate, and a connection unit  51   a , a communication unit  52   a  and an information storage unit  55   a  of the service provider  18  operate. The connection unit  51   a , the communication unit  52   a  and the information storage unit  55   a  have the same facilities as the equivalent units of the service provider  17 . 
     Using the IP address received from the computer  14  on the originating side shown in FIG. 5, the connection unit  51  of the service provider  17  on the originating side  17  establishes an IP-IP connection with the connection unit  51   a  of the service provider  18  on the receiving side (S 31 ). 
     The communication unit  52  notifies the communication unit  52   a  of the service provider  18  of a phone number and an IP address of the computer  14  on the originating side. Further, the communication unit  52  supplies a phone number and an IP address of the recipient to the communication unit  52   a  (S 32 ). 
     The communication unit  52   a  receiving the information from the communication unit  52  examines the information so as to determine whether the received phone number is registered in the information storage unit  55   a  (S 33 ). When the phone number is registered, the connection unit  51   a  performs a connection process with respect to the computer  15  on the receiving side. When the phone number is not registered, a request from the service provider  17  may be refused. The examination process of step  33  is not indispensable. 
     FIG. 15 shows an operation for transmitting and receiving information over the public network  16  between the service provider  18  on the receiving side and the computer  15  on the receiving side in the Internet telephone system shown in FIG.  5 . In transmission and reception of information between the service provider  18  and the computer  15 , the connection unit  51   a , the communication unit  52   a  and the information storage unit  55   a  of the service provider  18  operate, and the connection unit  31   a , the communication unit  34   a  and the information storage unit  39   a  of the computer  15  operate. 
     Using a recipient phone number provided by the service provider  17  on the originating side, the service provider  18  on the receiving side  18  dials the connection unit  31   a  of the computer  15  (S 41 ). 
     The communication unit  34   a  of the computer  15  reads out a user ID and a password registered in the information storage unit  39   a . For example, the communication unit  34   a  reads out a user ID ‘U02’, and a password ‘PSW02’ (S 42 ), and notifies the communication unit  52   a  of the service provider  18  accordingly (S 43 ). 
     The communication unit  52   a  receiving the information from the communication unit  34   a  examines the user ID and the password registered in the information storage unit  55   a  (S 44 ). When the user IDs match and the passwords match, the communication unit  52   a  permits an Internet connection. The communication unit  52   a  refuses a request from the call originator side when the user IDs and the passwords do not match. When the user ID and the password are authenticated by the service provider  18 , the communication unit  52   a  notifies the communication unit  34   a  of the phone number and the IP address of the call originator. When the received information is different from the information registered in the information storage unit  39   a , the communication unit  34   a  updates the registered information. 
     In this state, the computer  15  calls the telephone set  12  (S 45 ) so that a user on the receiving side can start a conversation using the Internet by taking a receiver of the telephone set  12 . 
     FIG. 16 shows an operation performed in the Internet telephone system shown in FIG. 5 for an automatic switch to a call using the ordinary public network  16  when it is not possible to guarantee a predetermined transmission rate in the call using the Internet  19 . For example, in an automatic switch from the Internet  19  to the public network  16 , the communication unit  34 , the monitoring unit  36  and the switching unit  37  of the computer  14  on the originating side operate, and the communication unit  52  of the service provider  17  on the originating side  17  operates. Further, the communication unit  34   a , the monitoring unit  36   a  and the switching unit  37   a  of the computer  15  on the receiving side operate. The communication unit  34   a , the monitoring unit  36   a  and the switching unit  37   a  provide the same facilities as the equivalent units of the computer  14 . In the service provider  18  on the receiving side, the communication unit  52   a  providing the same facilities as the communication unit  52  of the service provider  17  operates. 
     In a call using the Internet  19  between the telephone set  11  and the telephone set  12 , the Internet telephone system according to the invention ensures that the call is made through the communication unit  34  of the computer  14 , the communication unit  52  of the service provider  17 , the communication unit  52   a  of the service provider  18 , and the communication unit  34   a  of the computer  15  (S 51 ). 
     When the predetermined transmission rate on the Internet  19  fails to be guaranteed, making it difficult to hear a sound in the call, the Internet telephone system according to the invention automatically switches from the Internet  19  to the public network  16 , using the communication unit  34  and the communication unit  34   a , so that the call can be continued (S 52 ). 
     A switch between lines in the above step  52  is performed in the following procedures. The monitoring unit  36  monitors the transmission rate of data that the communication unit  34  processes in the communication. When a predetermined transmission rate fails to be guaranteed, the switching unit  37  sends control data instructing a switch from the Internet  19  to the public network  16  to the communication unit  34 , so that the communication unit  34  switches the lines. The switch between lines in step  52  is also performed in the computer  15 . The monitoring unit  36   a  monitors the transmission rate of data processed by the communication unit  34   a . When the predetermined transmission rate fails to be guaranteed, the switching unit  37   a  sends control data instructing a switch from the Internet  19  to the public network  16  to the communication unit  34   a  so that the communication unit  34   a  switches the line. In this way, switching between lines in the Internet telephone system according to the invention is started by the computer which has recognized that the transmission rate cannot be guaranteed. 
     FIG. 17 shows an operation performed in the Internet telephone system shown in FIG. 5 between the existing telephone set  13  not connected to a computer and the telephone set  11  connected to the computer  14 . In an operation between the telephone set  11  connected to the computer  14  and the existing telephone set  13  not connected to a computer, the connection unit  31 , the communication unit  34 , the registering unit  35 , and the information storage unit  39  of the computer  14  operate. 
     When the telephone set  13  is dialed by the telephone set  11  (S 61 ), the connection unit  31  of the computer  14  dials the telephone set  13  using a phone number from the telephone set  11  (S 62 ). 
     The communication unit  34  of the computer  14  examines the phone number information registered in the information storage unit  39  (S 63 ). The communication unit  34  demands an IP address from the telephone set  13  in case the phone number information of the telephone set  13  does not exist in the destination phone number information, or when the control information of the destination phone number information is ‘1’ (S 64 ). When the control information of the destination phone number information is set to ‘1’, it indicates that the call using the Internet is to be performed. When it is set to ‘0’, it indicates that the call using the Internet is not to be performed. 
     Because the telephone set  13  cannot recognize the IP address request in the case shown in FIG. 17, the IP address is not provided. The registering unit  35  registers the phone number of the telephone set  13  as the destination phone number information, when it is found that the destination phone number information is not registered in the information storage unit  39  as a result of the examination by the communication unit  34  (S 65 ). 
     Because the IP address request failed, the communication unit  34  recognizes the telephone set  13  as an ordinary telephone set not connected to a computer and supplies conversation data from the telephone set  11  to the telephone set  13  over the public network  16  as in any ordinary telephone call (S 66 ). 
     FIG. 18 shows the control executed in the Internet telephone system shown in FIG. 5 between the existing telephone set  13  not connected to a computer and the telephone set  11  connected to the computer  14 . 
     Because the computer  14  to which the telephone set  11  is connected cannot recognize whether a computer is connected to the destination telephone set  13 , it refers to the destination phone number managed by the information storage unit  39 . 
     When the destination phone number information is not registered or when the destination phone number information is registered and the control information is ‘1’, the computer  14  assumes that the destination (the telephone set  13 ) is a computer and uses the public network  16  so as to demand an IP address from the telephone set  13 , as shown in FIG.  18 . Simultaneously, the computer  14  notifies the telephone set  13  of a phone number and an IP address of the telephone set  11 . 
     However, the telephone set  13  cannot recognize the IP address request, the phone number and the IP address of the telephone set  11 . The communication unit  34  recognizes such a state as an NG state wherein a disconnection of a line and a time-out have occurred. In this case, the communication unit  34  of the computer  14  dials the telephone set  13  for a second time so as to place a call via the public network  16 . 
     FIG. 19 shows an operation performed in the Internet telephone system shown in FIG. 5 between the existing telephone set  13  not connected to a computer and the telephone set  11  connected to the computer  14 , showing an operation different from the operation of FIG.  17 . In an operation between the telephone set  11  connected to the computer  14  and the existing telephone set  13  not connected to a computer, the connection unit  31 , the communication unit  34  and the information storage unit  39  of the computer  14  operate. The operation of FIG. 19 differs from the operation of FIG. 17 in that the control information provided in correspondence to the destination phone number registered in the information storage unit  39  is set to ‘0’, indicating that the Internet is not used. 
     When the telephone set  11  dials the telephone set  13  (S 71 ), the connection unit  31  of the computer  14  dials the telephone set  13 , using a phone number provided by the telephone set  11  (S 72 ). 
     The communication unit  34  of the computer  14  reads out the destination phone number information registered in the information storage unit  39  (S 73 ). When the control information provided in correspondence to the destination phone number information is set to ‘0’, the communication unit  34  supplies conversation data from the telephone set  11  to the telephone set  13  over the public network  16  as in an ordinary telephone call, without demanding an IP address from the telephone set  13  (S 74 ). 
     FIG. 20 shows the control corresponding to FIG. 19 performed in the Internet telephone system shown in FIG. 5 between the existing telephone set  13  not connected to a computer and the telephone set  11  connected to the computer  14 . 
     Because the computer  14  connected to the telephone set  11  cannot recognize whether a computer is connected to the destination telephone set  13 , it examines the information storage unit  39 . 
     When the destination phone number information is registered and the control information is set to ‘0’, the computer  14  recognizes the destination (the telephone set  13 ) as being a telephone set not connected to a computer. The computer  14  uses the public network  16  to dial and performs a call over the public network  16 . 
     FIG. 21 shows an answer phone operation in the Internet telephone system shown in FIG.  5 . For example, in the answer phone operation, the connection unit  51  and the communication unit  52  of the service provider  17  on the originating side operate, and the connection unit  51   a , the communication unit  52   a , the registering unit  54   a  and the information storage unit  55   a  of the service provider  18  on the receiving side operate. The connection unit  51   a , the communication unit  52   a , the registering unit  54   a  and the information storage unit  55   a  have the same facilities as the respective equivalent units of the service provider  17 . 
     The computer  14  on the originating side dials the service provider  17  so as to perform a PPP connection (S 81 ). Subsequently, an IP-IP connection is performed between the connection unit  51   a  of the service provider  18  and the connection unit  51  of the service provider  17  (S 82 ). 
     The connection unit  51   a  of the service provider  18  tries connecting to the computer  15  on the receiving side (S 83 ). When the connection fails for the reason that a power supply of the computer  15  is not on, for example, the registering unit  54   a  registers conversation data supplied from the communication unit  52  of the service provider  17  to the communication unit  52   a  of the service provider  18  (S 84 ), in the information storage unit  55   a  (S 85 ). The information storage unit  55   a  manages the registered data as conversation data for the associated user. The conversation data registered in the information storage unit  55   a  can be monitored by a request from the computer  15 . 
     FIG. 22 shows an operation of analog-to-digital conversion in the Internet telephone system shown in FIG.  5 . In an operation of analog-to-digital conversion, the conversion unit  33  and the communication unit  34  of the computer  14  operate. The computer  15  in FIG. 5 provides similar facilities. 
     If the data on the line is analog data (S 91 ), the conversion unit  33  of the computer  14  subjects the analog data to analog-to-digital conversion so as to convert the same into digital data (S 93 ). If the data on the line is digital data (S 92 ), the digital data are output through the conversion unit  33  (S 93 ). 
     The communication unit  34  analyzes the digital data. The communication unit  34  is required to transmit the digital data to a call destination telephone set as conversation data, if the header is set to, for example, “1”, as shown in FIG.  22 . Accordingly, the communication unit  34  transmits the digital data to the conversion unit  33  (S 94 ). If, for example, the header of the digital data is set to “00”, as shown in FIG. 22, that is, if the digital data is control data specifying a request for switching between communication networks, an IP address request, an IP address notification or the like, the communication unit  34  executes the necessary control using the control data. 
     When a recipient telephone set is an analog telephone set  13   a , the conversion unit  33  receiving the digital data (conversation data) in step  94  converts the conversation data into analog data, and supplies the analog data to the recipient telephone set (S 95 ). When the recipient telephone set is a digital telephone set  13   b , the conversion unit  33  receiving the digital data (conversation data) in step  94  supplies the digital data to the digital telephone  13   b  set without converting the conversation data into analog data (S 96 ). Data from the analog telephone set  13   a  and the digital telephone set  13   b  is converted in a similar manner and transmitted to the destination telephone set. 
     FIG. 23 shows an operation for transmission and reception of information between the computer  14  and the computer  15  in the Internet telephone system shown in FIG. 5, performed when a main power supply of the destination computer  15  is on but the necessary hardware and software are not activated. In transmission and reception of information between the computer  14  and the computer  15 , the connection unit  31 , the communication unit  34  and the information storage unit  39  of the computer  14  operate, and the connection unit  31   a , the communication unit  34   a , the information storage unit  39   a  and the power-on unit  38   a  of the computer  15  operate. The communication unit  31   a , the communication unit  34   a , the information storage unit  39   a  and the power-on unit  38   a  provide the same facilities as the respective equivalent units of the computer  14 . The computer  14  shown in FIG. 5 is also provided with the power-on unit  38  having similar facilities. 
     In transmission and reception of information between the computer  14  and the computer  15 , if, for example, the telephone set  11  dials a telephone number when a power supply of the computer  15  is on but the necessary software and hardware are not activated (S 101 ), the connection unit  31  of the computer  14  dials a phone number of the destination computer  15  so as to access the connection unit  31   a  over the public network  16  (S 102 ). The connection unit  31   a  activates the power-on unit  38   a  (S 103 ), and the power-on unit  38   a  executes a process to activate the necessary hardware and software. 
     In FIG. 23, the power-on unit  38   a  activates the communication unit  34   a . The communication unit  34  of the computer  14  refers to the information managed by the information storage unit  39  (S 104 ), and supplies an IP address request to the communication unit  34   a  of the computer  15  (S 105 ). 
     The communication unit  34   a  supplied with the IP address request refers to the information managed by the information storage unit  39   a  (S 106 ), and transmits an IP address notification to the communication unit  34  of the computer  14  (S 107 ). When the operation of FIG. 23 is performed, it is required that each computer is capable of receiving information through a dialing process. When the dialing fails, the answer phone operation shown in FIG. 21 is executed. 
     FIG. 24 shows an operation for transmission and reception of information between the service provider  18  on the receiving side and the computer  15  on the receiving side, performed in the Internet telephone system shown in FIG.  5 . FIG. 24 shows an operation example different from that of FIG.  15 . In transmission and reception of information between the service provider  18  and the computer  15 , the connection unit  51   a , the communication unit  52   a  and the information storage unit  55   a  of the service provider  18  operate, and the connection unit  31   a , the communication unit  34   a , the information storage unit  39   a  and the power-on unit  38   a  of the computer  15  operate. The computer  14  shown in FIG. 5 is also provided with the power-on unit  38  having similar facilities. 
     Assuming that the computer  14  and the computer  15  are connected to each other over the Internet, a description will be given of transmission and reception of information performed when a power of the computer  15  is on but the necessary software and hardware are not activated, and when information relating to the recipient is already registered in the originating computer  14 . 
     When an IP-IP connection between the service provider  17  on the originating side and the service provider  18  on the receiving side is completed in this state, the connection unit  51   a  of the service provider  18  uses the public network  16  to dial the computer  15  on the receiving side (S 111 ). 
     The connection unit  31   a  of the computer  15  activates the power-on unit  38   a  (S 112 ) and activates the necessary hardware and software. In FIG. 24, the communication unit  34   a  is activated. 
     In this state, the communication unit  52   a  demands a user ID and a password for authenticating a user from the communication unit  34   a  of the computer  15  (S 113 ). The communication unit  34   a  receiving the notification refers to the information managed by the information storage unit  39   a  (S 114 ), and supplies the communication unit  52   a  of the service provider  18  with the user ID and the password (S 115 ). 
     The communication unit  52   a  receiving the information from the communication unit  34   a  examines the user ID and the password registered in the information storage unit  55   a  (S 116 ). The communication unit  52   a  permits an Internet connection when the user IDs match and the passwords match. When the user IDs and the passwords do not match, the communication unit  52   a  refuses a request from the call originator side. 
     When the service provider  18  gives an authentication, the phone number and the IP address of the call originator are supplied from the communication unit  52   a  to the communication unit  34   a . When it is found that the supplied information is different from the information that is registered in the information storage unit  39   a , the communication unit  34   a  updates the registered information. 
     In this state, the computer  15  calls the telephone set  12  (S 117 ), and a user on the receiving side starts a conversation using the Internet by operating the telephone set  12 . 
     FIG. 25 is a flowchart of an IP-type Internet telephone process executed in the Internet telephone system shown in FIG. 5, wherein the Internet  19  is used for IP-IP connection. 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  12  on the receiving side (S 201 ), the computer  14  on the originating side receiving the phone number searches the computer  14  for the phone number of the telephone set  12  on the receiving side. 
     When the phone number is not registered, the computer  14  uses the public network  16  to dial the phone number which the telephone set  11  on the originating side dialed and demands an IP address from the computer  15  on the receiving side (S 202 ). Simultaneously, the computer  14  supplies a phone number and an IP address from the originating side. Upon receiving the IP address request, the computer  15  stores the phone number and the IP address from the originating side, and supplies the IP address thereof to the computer  14  on the originating side over the public network  16  (S 203 ). The computer  14  on the originating side stores the received IP address and the phone number and disconnects itself from the public network  16  connected to the computer  15  on the receiving side. The computer  14  then enters an IP-IP connection with the IP address of the computer  15  on the receiving side over the Internet  19 , via the service provider  17  and the service provider  18  (S 204 ). 
     When it is determined in the dialing step  201  that the phone number of the telephone set  12  on the receiving side is registered in the computer  14 , the computer  14  on the originating side establishes an IP-IP connection with the IP address of the computer  15  on the receiving side over the Internet  19  via the service provider  17  and the service provider  18  (S 204 ). 
     When an IP-IP connection is completed, the computer  15  on the receiving side calls the telephone set  12  on the receiving side (S 205 ) so that the telephone set  11  and the telephone set  12  can start a call (S 206 ). 
     During a call, the computer  14  on the originating side and the computer  15  on the receiving side continue to monitor the transmission rate of conversation data on the Internet  19  to determine whether there is any abnormality in the transmission rate. The transmission rate of conversation data is preset by a user of the system. When the users of the telephone set  11  and the telephone set  12  execute an on-hook operation in this state (S 207 , S 208 ), the IP-IP connection is terminated and a call is ended (S 209 ). 
     For example, when an abnormality in the transmission rate is detected by the computer  14  on the originating side during a call, the computer  14  on the originating side transmits a network switching notification to the computer  15  on the receiving side (S 210 ), so that the IP-IP connection between the computers on the Internet  19  is terminated (S 211 ). When an abnormality in the transmission rate is detected by the computer  15  on the receiving side during a call, a similar operation is performed. That is, the computer  15  on the receiving side transmits a network switching notification to the computer  14  on the originating side (S 210 ) so as to terminate the IP-IP connection between the computers on the Internet  19  (S 211 ). 
     After the IP-IP connection is terminated, the computer  14  on the originating side dials the computer  15  on the receiving side over the public network  16  (S 212 ) so as to establish a connection therewith over the public network  16  (S 213 ). Thus, the users of the telephone set  11  and the telephone set  12  can talk with each other over the phone without being aware of the switch from the Internet  19  to the public network  16 . 
     It is assumed that the users of the telephone set  11  and the telephone set  12  perform an on-hook operation in this state (S 214 , S 215 ). If an IP-IP connection over the Internet  19  has been proceeding, the IP-IP connection is terminated (S 216 ). If the connection is via the public network  16 , the connection is terminated so that the call is ended. 
     FIG. 26 is a flowchart of a process of a dial-up Internet telephone system in which the Internet  19  is accessed by a dial-up connection. 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  12  on the receiving side (S 221 ), the computer on the originating side  14  receiving the phone number searches the computer  14  for the phone number of the telephone set  12  on the receiving side. 
     When the phone number is not registered, the computer  14  uses the public network  16  to dial the phone number which the telephone set  11  on the originating side dialed and demands an IP address from the computer  15  on the receiving side (S 222 ). Simultaneously, the computer  14  supplies a phone number and an IP address from the originating side. Upon receiving the IP address request, the computer  15  stores the phone number and the IP address from the originating side, and supplies the IP address thereof to the computer  14  on the originating side over the public network  16  (S 223 ). The computer  14  on the originating side stores the received IP address and the phone number and disconnects itself from the public network  16  connected to the computer  15  on the receiving side. The computer  14  dials the service provider  17 , so as to establish a connection with the service provider  17  over the public network  16  (S 224 ). 
     Simultaneously, the computer  14  on the originating side supplies the phone number and the IP address of the computer  15  on the receiving side to the service provider  17  on the originating side. Using the received IP address, the service provider  17  on the originating side establishes an IP-IP connection over the Internet  19  with the service provider  18  on the receiving side (S 225 ). The service provider  17  on the originating side also supplies the received phone number to the service provider  18  on the receiving side. Using the received phone number, the service provider  18  on the receiving side dials the computer  15  on the receiving side (S 226 ), so that the computer  15  on the receiving side calls the telephone set  12  when the connection with the service provider  18  is authenticated (S 227 ). In this state, the telephone set  11  and the telephone set  12  can start a speech communication (S 228 ). 
     When it is found that the phone number and the IP address of the telephone set  12  on the receiving side are registered in the computer  14  in the dialing step  221 , the computer  14  on the originating side dials the service provider  17  so as to connect to the service provider  17  over the public network  16  (S 224 ). Simultaneously, the computer  14  on the originating side notifies the service provider  17  on the originating side of the phone number and the IP address of the computer  15  on the receiving side. Using the received IP address, the service provider  17  on the originating side establishes an IP-IP connection over the Internet  19  with the service provider  18  on the receiving side (S 225 ), and supplies the received phone number to the service provider  18  on the receiving side. Using the received phone number, the service provider  18  on the receiving side dials the computer  15  on the receiving side (S 226 ). The computer  15  on the receiving side calls the telephone set  12  when the connection with the service provider  18  is authenticated (S 227 ). The telephone set  11  and the telephone set  12  can start a speech communication in this state (S 228 ). 
     During a call, the computer  14  on the originating side and the computer  15  on the receiving side continue to monitor the transmission rate of conversation data on the Internet  19  so as to determine whether there is any abnormality in the transmission rate. It is assumed that the transmission rate of conversation data is preset by a user of the system. 
     When the user of the telephone set  11  performs an on-hook operation in this state (S 229 ), the computer  14  terminates the connection with the service provider  17  over the public network  16  (S 231 ), and the service provider  17  terminates the IP-IP connection over the Internet  19  so that the call is ended (S 233 ). Similarly, when the user of the telephone set  12  performs an on-hook operation (S 230 ), the computer  15  terminates the connection between the computer  15  and the service provider  18  connected over the public network  16  (S 232 ), and the service provider  18  terminates the IP-IP connection so that the call is ended (S 233 ). 
     For example, when an abnormality in the transmission rate is detected by the computer  14  on the originating side during a call, the computer  14  on the originating side transmits a network switching notification to the computer  15  on the receiving side (S 234 ), so that the connection between the computer  14  and the service provider  17  connected over the public network  16  is terminated (S 235 ). Further, the service provider  17  terminates the IP-IP connection over the Internet  19  (S 237 ). When an abnormality in the transmission rate is detected by the computer  15  on the receiving side during a call, a similar process is performed. That is, the computer  15  on the receiving side terminates the connection between the computer  15  and the service provider  18  connected over the public network  16  (S 236 ). Further, the service provider  18  terminates the IP-IP connection over the Internet  19  (S 237 ). 
     When the IP-IP connection is terminated, the computer  14  on the originating side dials the computer  15  on the receiving side over the public network  16  (S 238 ) so as to establish a connection therewith over the public network  16  (S 239 ). Thus, the users of the telephone set  11  and the telephone set  12  can talk to each other over the phone without being aware of the switch from the Internet  19  to the public network  16 . 
     It is assumed that the users of the telephone set  11  and the telephone set  12  perform an on-hook operation in this state (S 240 , S 241 ). If an IP-IP connection over the Internet  19  has been proceeding, the IP-IP connection is terminated (S 242 ). If the connection is via the public network  16 , the connection is terminated (S 242 ) so that the call is ended. 
     FIG. 27 is a flowchart of a process of a dial-up Internet telephone system in which the Internet  19  is accessed by a dial-up connection, different from the process of FIG.  26 . 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  12  on the receiving side (S 251 ), the computer on the originating side  14  receiving the phone number searches the computer  14  for the phone number of the telephone set  12  on the receiving side. 
     When the phone number is not registered, the computer  14  uses the public network  16  to dial the phone number which the telephone set  11  on the originating side dialed and demands an IP address from the computer  15  on the receiving side (S 252 ). Simultaneously, the computer  14  supplies a phone number and an IP address from the originating side. Upon receiving the IP address request, the computer  15  stores the phone number and the IP address from the originating side, and supplies the IP address thereof to the computer  14  on the originating side over the public network  16  (S 253 ). The computer  14  on the originating side stores the received IP address and the phone number and disconnects itself from the public network  16  connected to the computer  15  on the receiving side. The computer  14  dials the service provider  17 , so as to establish a connection with the service provider  17  over the public network  16  (S 254 ). 
     Simultaneously, the computer  14  on the originating side supplies the phone number and the IP address of the computer  15  on the receiving side to the service provider  17  on the originating side. Using the received IP address, the service provider  17  on the originating side establishes an IP-IP connection over the Internet  19  with the service provider  18  on the receiving side (S 255 ). The computer  15  on the receiving side dials the service provider  18  on the receiving side (S 256 ), so that the service provider  18  on the receiving side relates the information received earlier from the service provider  17  on the originating side to the information from the computer  15  on the receiving side. When the connection with the service provider  18  is authenticated, the computer  15  on the receiving side calls the telephone set  12  (S 257 ). The telephone set  11  and the telephone set  12  can start a speech communication in this state (S 258 ). 
     When it is found that the phone number and the IP address of the telephone set  12  on the receiving side are registered in the computer  14  in the dialing step  251 , the computer  14  on the originating side dials the service provider  17  so as to connect to the service provider  17  over the public network  16  (S 254 ). Simultaneously, the computer  14  on the originating side notifies the service provider  17  on the originating side of the phone number and the IP address of the computer  15  on the receiving side. Using the received IP address, the service provider  17  on the originating side establishes an IP-IP connection over the Internet  19  with the service provider  18  on the receiving side (S 225 ), and supplies the received phone number to the service provider  18  on the receiving side. The computer  15  on the receiving side dials the service provider  18  on the receiving side (S 256 ), and the service provider  18  on the receiving side relates the information received earlier from the service provider  17  on the originating side to the information from the computer  15  on the receiving side. When the connection with the service provider  18  is authenticated, the computer  15  on the receiving side calls the telephone set  12  (S 257 ). The telephone set  11  and the telephone set  12  can start a speech communication in this state (S 258 ). 
     During a call, the computer  14  on the originating side and the computer  15  on the receiving side continue to monitor the transmission rate of conversation data on the Internet  19  so as to determine whether there is any abnormality in the transmission rate. It is assumed that the transmission rate of conversation data is preset by a user of the system. 
     When the user of the telephone set  11  performs an on-hook operation in this state (S 259 ), the computer  14  terminates the connection with the service provider  17  over the public network  16  (S 261 ), and the service provider  17  terminates the IP-IP connection over the Internet  19  so that the call is ended (S 263 ). Similarly, when the user of the telephone set  12  performs an on-hook operation (S 260 ), the computer  15  terminates the connection between the computer  15  and the service provider  18  connected over the public network  16  (S 262 ), and the service provider  18  terminates the IP-IP connection so that the call is ended (S 263 ). 
     For example, when an abnormality in the transmission rate is detected by the computer  14  on the originating side during a call, the computer  14  on the originating side transmits a network switching notification to the computer  15  on the receiving side (S 264 ), so that the connection between the computer  14  and the service provider  17  connected over the public network  16  is terminated (S 265 ). Further, the service provider  17  terminates the IP-IP connection over the Internet  19  (S 267 ). When an abnormality in the transmission rate is detected by the computer  15  on the receiving side during a call, a similar process is performed. That is, the computer  15  on the receiving side terminates the connection between the computer  15  and the service provider  18  connected over the public network  16  (S 266 ). Further, the service provider  18  terminates the IP-IP connection over the Internet  19  (S 267 ). 
     When the IP-IP connection is terminated, the computer  14  on the originating side dials the computer  15  on the receiving side over the public network  16  (S 268 ) so as to establish a connection therewith over the public network  16  (S 269 ). Thus, the users of the telephone set  11  and the telephone set  12  can talk to each other over the phone without being aware of the switch from the Internet  19  to the public network  16 . 
     It is assumed that the users of the telephone set  11  and the telephone set  12  perform an on-hook operation in this state (S 270 , S 271 ). If an IP-IP connection over the Internet  19  has been proceeding, the IP-IP connection is terminated (S 272 ). If the connection is via the public network  16 , the connection is terminated (S 272 ) so that the call is ended. 
     FIG. 28 is a flowchart of a process performed between the existing telephone set and a dial-up Internet telephone system in which the Internet  19  is accessed by a dial-up connection. 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  13  on the receiving side (S 281 ), the computer  14  on the originating side searches the computer  14  for the phone number of the telephone set  13  on the receiving side. 
     When the phone number is not registered, or when the phone number is registered and information indicating that the Internet is to be used is provided in relation to the registered phone number, the computer  14  on the originating side dials the previously received phone number over the public network  16  to access the telephone set  13  on the receiving side, and requests an IP address from the telephone set  13  (S 282 ). Simultaneously, the computer  14  supplies the phone number and the IP address from the originating side. Because the telephone set  13  receiving the IP address request cannot recognize the request, it rejects the request by not returning a response, for example (S 283 ). 
     If the phone number is registered, and information indicating that the Internet is not to be used is provided in relation to the registered phone number, the computer  14  on the originating side recognizes that the telephone set  13  on the receiving side is an ordinary telephone set to which a computer is not connected to, whereupon the computer  14  performs a dialing process over the public network  16  (S 284 ). 
     In this state, the telephone set  11  and the telephone set  13  can start an ordinary telephone call over the public network  16  (S 285 ). When the user of the telephone set  11  on the originating side and the user of the telephone set  13  on the receiving side perform an on-hook operation during a call (S 286 ), the connection over the public network  16  is terminated (S 287 ). 
     FIG. 29 shows a flowchart of a process performed on the originating side of the dial-up IP Internet telephone system in which the Internet  19  is accessed by a dial-up connection. 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  12  on the receiving side (S 301 ), the computer  14  on the originating side receiving the phone number searches the computer  14  to determine whether the recipient phone number and the recipient IP address are registered in the computer  14  (S 302 ). 
     When the phone number and the IP address are not registered (S 302 , NO), the computer  14  on the originating side uses the public network  16  to dial the phone number which the telephone set  11  on the originating side dialed, and demands an IP address from the computer  15  on the receiving side (S 303 ). Simultaneously, the computer  14  supplies the phone number and the IP address from the originating side. 
     When the IP address and the phone number from the computer  15  are supplied to the computer  14  on the originating side over the public network  16  as a reply to the IP address request, the computer  14  on the originating side stores the received IP address and phone number (S 304 ), and disconnects itself from the public network  16  connected to the computer  15  on the receiving side (S 305 ). Further, the computer  14  establishes an IP-IP connection with the IP address of the computer  15  on the receiving side over the Internet  19 , via the service provider  17  and the service provider  18  (S 306 ). 
     When it is determined in the search step  302  that the phone number and the IP address of the telephone set  12  on the receiving side are registered in the computer  14  (S 302 , YES), the computer  14  on the originating side establishes an IP-IP connection over the Internet  19  with the IP address of the computer  15  on the receiving side, via the service provider  17  and the service provider  18  (S 306 ). 
     When the IP-IP connection is completed, the computer  15  on the receiving side calls the telephone set  12  on the receiving side. In this state, the telephone set  11  and the telephone set  12  can start a telephone call. 
     It is assumed that a user of the telephone set  11  performs an on-hook operation while a call is proceeding (S 307 , YES). When a call using the Internet  19  is proceeding (S 315 , YES), the computer  14  on the originating side terminates the IP-IP connection (S 316 ) so as to end the call. When a call using the public network  16  is proceeding (S 315 , NO), the computer  14  on the originating side terminates the connection with the public network  16  (S 317 ) so as to end the call. 
     The computer  14  on the originating side continues to monitor the transmission rate of conversation data on the Internet  19  to determine whether there is any abnormality in the transmission rate (S 308 ). It is assumed that the transmission rate of conversation data is preset by the user of the system. 
     For example, when the computer  14  on the originating side has detected an abnormality in the transmission rate during a call (S 307 , NO) (S 308 , YES), the telephone set  14  on the originating side transmits a network switch notification to the computer  15  on the receiving side (S 311 ) and terminates the IP-IP connection between the computers on the Internet  19  (S 312 ). 
     After terminating the IP-IP connection, the computer  14  on the originating side dials the computer  15  on the receiving side over the public network  16  in (S 313 ) so as to establish a connection therewith over the public network  16 . Thus, the user of the telephone set  11  can make a telephone call without being aware of the switch from the Internet  19  to the public network  16 . 
     For example, when the computer  15  on the receiving side detects an abnormality in the transmission rate during a call, and the computer  14  receives a network switch notification from the computer  15  (S 307 , NO) (S 308 , NO) (S 309 , YES) (S 314 ), the telephone set  14  on the originating side stores the information relating to the network switching. 
     When the user of the telephone set  11  does not perform an on-hook operation during a call (S 307 , NO), when the computer  14  on the originating side does not detect an abnormality in the transmission rate during the call (S 308 , NO) and when the network switch notification from the computer  15  is not received during the call (S 309 , NO), the computer  14  on the originating side maintains the call state (S 310 ). 
     FIG. 30 is a flowchart of a process performed on the receiving side of an IP Internet telephone system in which the Internet  19  is accessed by an IP-IP connection. 
     When the computer  15  on the receiving side receives a dialing signal from the telephone set  11  on the originating side (S 321 ), the computer  15  on the receiving side receives an IP address request, a phone number and an IP address from the originating side (S 322 ). The computer  15  on the receiving side receiving the IP address request supplies a phone number and an IP address on the receiving side to the computer  14  on the originating side as a reply (S 323 ). 
     The computer  15  on the receiving side receiving the phone number and the IP address from the originating side searches itself to determine whether the received information is registered in the computer  15  (S 324 ). 
     When the phone number and the IP address are not registered (S 324 , NO), the computer  15  on the receiving side registers the phone number and the IP address from the originating side (S 325 ). The computer  15  then terminates the connection with the public network  16  connected to the computer  14  on the originating side (S 326 ) and waits for an IP-IP connection over the Internet  19  with the computer  14  on the originating side (S 327 ). 
     When it is determined in step  324  that the phone number and the IP address from the originating side are registered in the computer  15  (S 324 , YES), the computer  15  on the receiving side waits for an IP-IP connection over the Internet  19  with the computer  14  on the originating side (S 327 ). 
     When the IP-IP connection is completed, the computer  15  on the receiving side calls the telephone set  12  on the receiving side. In this state, the telephone set  11  and the telephone set  12  can start a call. 
     It is assumed that the user of the telephone set  12  performs an on-hook operation during a call (S 328 , YES). When a call using the Internet  19  is proceeding (S 336 , YES), the computer  15  on the receiving side terminates the IP-IP connection (S 337 ) so as to end the call. When a call using the public network  16  is proceeding (S 336 , No), the computer  15  on the receiving side terminates the connection with the public network  16  (S 338 ) so as to end the call. 
     The computer  15  on the receiving side continues to monitor the transmission rate of conversation data on the Internet  19  so as to determine whether there is any abnormality in the transmission rate (S 329 ). It is assumed that the transmission rate of conversation data is preset by the user of the system. 
     For example, when the computer  14  on the originating side has detected an abnormality in the transmission rate during a call (S 328 , NO) (S 329 , YES), the telephone set  15  on the receiving side transmits a network switch notification to the computer  14  on the originating side (S 332 ) and terminates the IP-IP connection between the computers on the Internet  19  (S 333 ). 
     After terminating the IP-IP connection, the computer  15  on the receiving side waits for a dialing signal from the computer  14  on the originating side (S 334 ), and when the dialing signal arrives from the computer  14  on the originating side, the computer  15  establishes a connection therewith over the public network  16 . Thus, the user of the telephone set  12  can receive a telephone call without being aware of the switch from the Internet  19  to the public network  16 . 
     For example, when the computer  14  on the originating side detects an abnormality in the transmission rate during a call, and when the computer  15  receives the network switch notification from the computer  14  (S 328 , NO) (S 329 , NO) (S 330 , YES), the telephone set  15  on the receiving side stores information relating to network switching (S 335 ). 
     When the user of the telephone set  12  does not perform an on-hook operation during a call (S 328 , NO), when the computer  15  on the receiving side does not detect an abnormality in the transmission rate during the call (S 329 , NO), and when the network switch notification from the computer  14  is not received during the call (S 330 , NO), the computer  15  on the receiving side maintains the call state (S 331 ). 
     FIG. 31 is a flowchart of a process performed on the originating side of a dial-up Internet telephone system in which the Internet  19  is accessed by a dial-up connection. 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  12  on the receiving side (S 341 ), the computer  14  on the originating side receiving the phone number searches itself to determined whether the received phone number and IP address are registered in the computer  14  (S 342 ). 
     When the phone number and the IP address are not registered (S 342 , NO), the computer  14  on the originating side uses the public network  16  to dial the phone number that the telephone set  11  on the originating side dialed and requests an IP address from the computer  15  on the receiving side (S 343 ). Simultaneously, the computer  14  supplies a phone number and an IP address from the originating side. 
     When the IP address and the phone number from the computer  15  are supplied to the computer  14  on the originating side over the public network  16  in reply to the IP address request, the computer  14  on the originating side stores the received IP address and phone number (S 344 ). The computer  14  then terminates the connection with the public network  16  connected to the computer  15  on the receiving side (S 345 ) and dials the service provider  17  so as to connect to the service provider  17  over the public network  16  (S 346 ). Simultaneously, the computer  14  on the originating side supplies a phone number and an IP address of the computer  15  on the receiving side to the service provider  17  on the transmitting side. 
     The service provider  17  on the originating side establishes an IP-IP connection to the service provider  18  on the receiving side over the Internet  19 , using the received IP address, and supplies the received phone number to the service provider  18  on the receiving side. The service provider  18  on the receiving side dials the computer  15  on the receiving side using the received phone number so that the computer  15  on the receiving side calls the telephone set  12  when the connection with the service provider  18  is authenticated. 
     When it is determined in step  342  that the phone number and the IP address of the telephone set  12  on the receiving side are registered in the computer  14  (S 342 , YES), the computer  14  on the originating side dials the service provider  17  so as to connect to the service provider  17  over the public network  16  (S 346 ). Simultaneously, the computer  14  on the originating side supplies a phone number and an IP address of the computer  15  on the receiving side to the service provider  17  on the originating side. 
     The service provider  17  on the originating side establishes an IP-IP connection to the service provider  18  on the receiving side over the Internet  19  using the received IP address, and supplies the received phone number to the service provider  18  on the receiving side. The service provider  18  on the receiving side dials the computer  15  on the receiving side using the received phone number so that the computer  15  on the receiving side calls the telephone set  12  when the connection with the service provider  18  is authenticated. 
     When the IP-IP connection is completed, the computer  15  on the receiving side calls the telephone set  12  on the receiving side. In this state, the telephone set  11  and the telephone set  12  can start a call. 
     It is assumed that, while a call is proceeding, the user of the telephone set  11  performs an on-hook operation (S 347 , YES). When a call using the Internet  19  is proceeding (S 356 , YES), the computer  14  on the originating side supplies a request to release the IP-IP connection to the service provider  17  on the originating side (S 357 ). The service provider  17  on the originating side receiving the request to release the IP-IP connection terminates the IP-IP connection (S 358 ) so as to end the call. When a call using the public network  16  is proceeding (S 356 , NO), the computer  14  on the originating side terminates the connection with the public network  16  (S 359 ) so as to end the call. 
     The computer  14  on the originating side continues to monitor the transmission rate of conversation data on the Internet  19  to determine whether there is any abnormality in the transmission rate (S 348 ). It is assumed that the transmission rate of conversation data is preset by the user of the system. 
     For example, when the computer  14  on the originating side has detected an abnormality in the transmission rate during a call (S 347 , NO) (S 348 , YES), the telephone set  14  on the originating side transmits a network switch notification to the computer  15  on the receiving side (S 351 ) and supplies a request to release the IP-IP connection to the service provider  17  on the originating side (S 352 ). The service provider  17  on the originating side receiving the request to release the IP-IP connection terminates the IP-IP connection (S 353 ). 
     After terminating the IP-IP connection, the computer  14  on the originating side dials the computer  15  on the receiving side over the public network  16  (S 354 ) and establishes a connection therewith over the public network  16 . Thus, the user of the telephone set  11  can make a telephone call without being aware of the switch from the Internet  19  to the public network  16 . 
     For example, when the computer  15  on the receiving side has detected an abnormality in the transmission rate during a call and the computer  14  has received a network switch notification from the computer  15  (S 347 , NO) (S 348 , NO) (S 349 , YES), the telephone set  14  on the originating side stores information relating to the network switching (S 355 ). 
     When the user of the telephone set  11  does not perform an on-hook operation during a call (S 347 , NO), when the computer  14  on the originating side does not detect an abnormality in the transmission rate during the call (S 348 , NO) and when the network switch notification from the computer  15  is not received during the call (S 349 , NO), the computer  14  on the originating side maintains the call state (S 350 ). 
     FIG. 32 is a flowchart of a process performed on the receiving side of a dial-up Internet telephone system in which the Internet  19  is accessed by a dial-up connection. 
     When the computer  15  on the receiving side receives a dialing signal from the telephone set  11  on the originating side (S 361 ), the computer  15  on the receiving side receives an IP address request, a phone number and an IP address from the originating side (S 362 ). The computer  15  on the receiving side receiving the IP address request supplies the received phone number and IP address to the computer  14  on the originating side as a reply (S 363 ). 
     The computer  15  on the receiving side receiving the phone number and IP address from the originating side searches itself to determine whether the received information is registered in the computer  15  (S 364 ). 
     When the phone number and the IP address are not registered (S 364 , NO), the computer  15  on the receiving side registers the phone number and the IP address from the originating side (S 365 ), terminates the connection with the public network  16  connected to the computer  14  on the originating side (S 36 &amp;), and waits for an IP-IP connection over the Internet  19  with the computer  14  on the originating side (S 367 ). When the recipient phone number is received from the service provider  17  on the originating side, the service provider  18  on the receiving side dials the computer  15  on the receiving side using the received phone number so that the computer  15  on the receiving side calls the telephone set  12  when the connection with the service provider  18  is authenticated. In this state, the telephone set  11  and the telephone set  12  can start a call. 
     When it is determined in step  364  that the phone number and the IP address from the originating side are registered in the computer  15  (S 364 , YES), the computer  15  on the receiving side waits for an IP-IP connection over the Internet  19  with the computer  14  on the originating side (S 367 ). When the recipient phone number is received from the service provider  17  on the originating side, the service provider  18  on the receiving side dials the computer  15  on the receiving side using the received phone number so that the computer  15  on the receiving side calls the telephone set  12  when connection with the service provider  18  is authenticated. 
     When the IP-IP connection is completed, the computer  15  on the receiving side calls the telephone set  12  on the receiving side. In this state, the telephone set  11  and the telephone set  12  can start a call. 
     It is assumed that, while a call is proceeding, the user of the telephone set  12  performs an on-hook operation (S 368 , YES). 
     When a call using the Internet  19  is proceeding (S 377 , YES), the computer  15  on the receiving side supplies a request to release the IP-IP connection to the service provider  18  on the receiving side (S 378 ). The service provider  18  on the receiving side receiving the request to release the IP-IP connection terminates the IP-IP connection (S 379 ) so as to end the call. When a call using the public network  16  is proceeding (S 377 , NO), the computer  15  on the receiving side terminates the connection with the public network  16  (S 380 ) so as to end the call. 
     The computer  15  on the receiving side continues to monitor the transmission rate of conversation data on the Internet  19  to determine whether there is any abnormality in the transmission rate (S 369 ). It is assumed that the transmission rate of conversation data is preset by the user of the system. 
     For example, when the computer  15  on the receiving side has detected an abnormality in the transmission rate during a call (S 368 , NO) (S 369 , YES), the telephone set  12  on the receiving side transmits a network switch notification to the computer  14  on the originating side (S 372 ) and supplies a request to release the IP-IP connection to the service provider  18  on the receiving side (S 373 ). The service provider  18  on the receiving side receiving the request to release the IP-IP connection terminates the IP-IP connection (S 374 ). 
     After the IP-IP connection is terminated, the computer  15  on the receiving side waits for a dialing signal from the computer  14  on the originating side (S 375 ), and when the dialing signal arrives from the computer  14  on the originating side, it establishes a connection therewith over the public network  16 . Thus, the user of the telephone set  12  can receive a telephone call without being aware of the switch from the Internet  19  to the public network  16 . 
     For example, when the computer  14  on the originating side detects an abnormality in the transmission rate during a call, and when the computer  15  receives a network switch notification from the computer  14  (S 368 , NO) (S 369 , NO) (S 370 , YES), the telephone set  15  on the receiving side stores information relating to the network switching (S 376 ). 
     When the user of the telephone set  12  does not perform an on-hook operation during a call (S 368 , NO), when the computer  15  on the receiving side does not detect an abnormality in the transmission rate during the call (S 369 , NO) and when the network switch notification from the computer  14  is not received during the call (S 370 , NO), the computer  15  on the receiving side maintains the call state (S 371 ). 
     FIG. 33 is a flowchart of a process performed on the receiving side of a dial-up Internet telephone system in which the Internet  19  is accessed by a dial-up connection, showing a process different from the process of FIG.  32 . 
     When the computer  15  on the receiving side receives a dialing signal from the telephone set  11  on the originating side (S 391 ), the computer  15  on the receiving side receives an IP address request, and a phone number and an IP address from the originating side (S 392 ). The computer  15  on the receiving side receiving the IP address request supplies a phone number and an IP address on the receiving side to the computer  14  on the originating side as a reply (S 393 ). 
     The computer  15  on the receiving side receiving the phone number and the IP address from the originating side searches itself to determine whether the received information is registered in the computer  15  (S 394 ). 
     When the phone number and the IP address are not registered (S 394 , NO), the computer  15  on the receiving side registers the phone number and the IP address from the originating side (S 395 ), and terminates the connection with the public network  16  connected to the computer  14  on the originating side (S 396 ). The computer  15  on the receiving side dials the service provider  18  on the receiving side (S 397 ) so that the service provider  18  on the receiving side compares the information previously received from the service provider  17  on the originating side and the information from the computer  15  on the receiving side. When the connection with the service provider  18  is authenticated, the computer  15  on the receiving side calls the telephone set  12 . 
     When it is determined in step  364  that a phone number and an IP address from the originating side are registered in the computer  15  (S 394 , YES), the computer  15  on the receiving side dials the service provider  18  on the receiving side (S 397 ) so that the service provider  18  on the receiving side compares the information previously received from the service provider  17  on the originating side and the information from the computer  15  on the receiving side. When the connection with the service provider  18  is authenticated, the computer  15  on the receiving side calls the telephone set  12 . 
     When the IP-IP connection is completed, the computer  15  on the receiving side calls the telephone set  12  on the receiving side. In this state, the telephone set  11  and the telephone set  12  can start a call. 
     It is assumed that the user of the telephone set  12  performs an on-hook operation while a call is proceeding (S 398 , YES). When a call using the Internet  19  is proceeding (S 407 , YES), the computer  15  on the receiving side supplies a request to release the IP-IP connection to the service provider  18  on the receiving side (S 408 ). The service provider  18  on the receiving side receiving the request to release the IP-IP connection terminates the IP-IP connection (S 409 ) so as to end the call. When a call using the public network  16  is proceeding (S 407 , NO), the computer  15  on the receiving side terminates the connection over the public network  16  (S 410 ) so as to end the call. 
     The computer  15  on the receiving side continues to monitor the transmission rate of conversation data on the Internet  19  to determine whether there is any abnormality in the transmission rate (S 399 ). It is assumed that the transmission rate of conversation data is preset by the user of a system. 
     For example, when the computer  15  on the receiving side has detected an abnormality in the transmission rate during a call (S 398 , NO) (S 399 , YES), the telephone set  15  on the receiving side transmits a network switch notification to the computer  14  on the originating side (S 402 ) and supplies a request to release the IP-IP connection to the service provider  18  on the receiving side (S 403 ). The service provider  18  on the receiving side receiving the request to release the IP-IP connection terminates the IP-IP connection (S 404 ). 
     After terminating the IP-IP connection, and the computer  15  on the receiving side waits for a dialing signal from the computer  14  on the originating side (S 405 ), and when the dialing signal arrives from the computer  14  on the originating side, it establishes a connection over the public network  16 . Thus, the user of the telephone set  12  can receive a telephone call without being aware of the switch from the Internet  19  to the public network  16 . 
     For example, when the computer  14  on the originating side detects an abnormality in the transmission rate during a call, and when the computer  15  receives a network switch notification from the computer  14  (S 398 , NO) (S 399 , NO) (S 400 , YES), the telephone set  15  on the receiving side stores information relating to the network switching (S 406 ). 
     When the user of the telephone set  12  does not perform an on-hook operation during a call (S 398 , NO), when the computer  15  on the receiving side does not detect an abnormality in the transmission rate during the call (S 399 , NO) and when the network switch notification from the computer  14  is not received during the call (S 400 , NO), the computer  15  on the receiving side maintains the call state (S 401 ). 
     FIG. 34 is a flowchart of a process performed between a dial-up type Internet telephone system in which the Internet  19  is accessed by a dialup connection, and the existing telephone set to which a computer is not connected. 
     When the telephone set  11  on the originating side dials a phone number of the telephone set  13  on the receiving side (S 421 ), the computer  14  on the originating side examines itself to determine whether information indicating that the Internet is to be used is registered in the computer  14  and whether a call destination phone number and an IP address are registered in the computer  14  (S 422 ). 
     When the destination phone number is not registered, or when the destination phone number is registered and, at the same time, the information indicating that the Internet is to be used is provided in correspondence to the registered phone number, the computer  14  on the originating side dials the previously received recipient phone number so as to request an IP address from the telephone set  13  on the receiving side over the public network  16  (S 423 ). Simultaneously, the computer  14  supplies a phone number and an IP address from the originating side. Because the telephone set  13  on the receiving side cannot recognize the IP address request which it receives, it rejects the request by, for example, not returning a response. In this case, the computer  14  on the originating side recognizes that the telephone set  13  is an ordinary telephone set not connected to a computer, and performs a dialing operation over the public network  16  for a second time (S 424 ). 
     When the destination phone number is registered and the information indicating that the Internet is not to be used is provided in correspondence to the registered phone number, the computer  14  on the originating side recognizes that the telephone set  13  on the receiving side is an ordinary telephone set to which a computer is not connected, and performs a dialing operation over the public network  16  (S 425 ). 
     In this state, the telephone set  11  and telephone set  13  can start an ordinary telephone call over the public network  16  (S 426 , NO) (S 427 ). When the user of the telephone set  11  on the originating side or the user of the telephone set  13  on the receiving side performs an on-hook operation during a call (S 426 , YES), the connection over the public network  16  is terminated (S 428 ). 
     According to the Internet telephone system of the present invention, a telephone set on the originating side can make a telephone call to a recipient telephone set, by an operation similar to the ordinary operation for using a telephone set. Information of the telephone set need not be registered in a computer on the network managing the system. 
     According to the Internet telephone system of the present invention, long-distance calls can be made at a relatively low cost because the user uses the Internet as a communication network. 
     According to the Internet telephone system of this invention, the telephone operation performed by the user is similar to the operation performed when the user uses the public network. Therefore, the user need not be aware of different communication networks in use. 
     FIG. 3 shows a construction of a computer which implements the communication device of the Internet telephone system according to the invention. 
     Referring to FIG. 3, the computer comprises a control unit  301  which includes a CPU, a memory unit  302 , a display unit  303 , an input unit  304 , a CD-ROM drive unit  305  and a disk unit  306 . These units are connected to each other via a system bus A. The control unit  301  executes a program which stores processes of the Internet telephone system. 
     The memory unit  302  includes a memory such as a RAM or a ROM, and stores the program to be executed by the control unit  301  and the data obtained in the process of execution. 
     The display unit  303  is implemented by a CRT or an LCD (liquid crystal display panel) to display various information. 
     The input unit  304  is implemented by a keyboard, a mouse or the like. 
     A program for executing the process of the Internet telephone system is stored in a CD-ROM  400 . 
     A program is installed in the disk unit  306  from the CD-ROM  400  set in the CD-ROM drive unit  305 . The program is read out from the disk unit  306  when the computer is started and is stored in the memory unit  302 . In this state, the control unit  301  (CPU) executes the process of the Internet telephone system, according to the program stored memory unit  302 . 
     The CD-ROM  400  contains the program for the process of the Internet telephone system concerned, but the storage medium for storing the program need not be limited to a CD-ROM. Depending on the specification of a computer constituting the system, other types of recording medium such as a magnetic disk like a floppy disk, a MO disk, or a magnetic tape may also be used. 
     According to the Internet telephone system of the present invention, an originating user can make a telephone call to a recipient user, by an operation similar to the operation for using an ordinary telephone set. Information of the originating user need not be registered in a computer on the network managing the system. 
     In further accordance with the Internet telephone system of the present invention, long-distance calls can be made at a relatively low cost because the user uses the Internet as a communication network. 
     In further accordance with the Internet telephone system of this invention, the telephone operation performed by the user is similar to the operation performed when the user uses the public network. Therefore, the user need not be aware of different communication networks in use. 
     The present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the scope of the present invention.