Patent Publication Number: US-2006018267-A1

Title: IP telephone system, ENUM server and method for performing telephone conference

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an IP telephone system, an ENUM server and a method for performing a telephone conference that enable voice communication via an IP network.  
      2. Description of Related Art  
      The recent rapid growth of the Internet has drawn attention to an IP telephone system that enables low-cost voice communications with telephone apparatuses at remote places as well as free voice communications between IP telephone apparatuses owned by subscribers. There has been a request for conducting, for example, a company conference using such an IP telephone system. Such a conference usually involves a plurality of participants.  
      Recently, ENUM technology has drawn attention due to its ability to effectively manage information used for diversified communications methods including telephones, facsimiles, cellular phones, and e-mails and to enable various communication according to individual circumstances. ENUM is designed to identify the Internet service with a unique global identification number such as the E.164 number, using the DNS (Domain Name System). Currently, the IETF (Internet Engineering Task force) is seeking to standardize ENUM platforms, where protocol specifications are discussed (See Publication 1, for example).  
      [Publication 1] Issued by ENUM Trial Japan “ENUM Trial Japan First Report” May, 2004  
      However, the above-described system is problematic in that when the user has, for example, a company conference using the IP telephone system, it is necessary to individually provide information such as telephone numbers of other participants. This has been a problem in terms of extra labor required to prepare for a conference.  
      Particularly, when a number of participants are scheduled to attend a conference, a great amount of labor is required to prepare for a conference.  
     SUMMARY OF THE INVENTION  
      The present invention is provided to address the above-described problems. The purpose of the present invention is to provide an IP telephone system, an ENUM server and a method for performing a telephone conference that reduce labor required to prepare for a telephone conference and ensure smooth conference operation.  
      The present invention relates to an IP telephone system that includes a plurality of IP telephone apparatuses and an ENUM server. Those IP telephone apparatuses are connected to an IP network. The ENUM server stores a NAPTR resource record specifying a number which identifies a telephone conference being conducted between these IP telephone apparatuses and specifying identification data for an IP telephone apparatus that attends the telephone conference, and transmits a corresponding NAPTR resource record in response to a query from an IP telephone apparatus. In the IP telephone system, the IP telephone apparatus that attends the telephone conference transmits, to the ENUM server, a query for a NAPTR resource record by specifying the number which identifies the telephone conference. The IP telephone apparatus then obtains identification data for another IP telephone apparatus that attends the telephone conference based on the received NAPTR resource record. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is further described in the detailed description which follows, with reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:  
       FIG. 1  illustrates a network configuration to which an IP telephone system according to an embodiment of the present invention is applied;  
       FIG. 2  illustrates an example of a NAPTR record corresponding to a conference number stored in a DB of an ENUM server in the IP telephone system according to the embodiment;  
       FIG. 3  illustrates a functional block diagram for a mixing server in the IP telephone system according to the embodiment;  
       FIG. 4  illustrates a diagram describing a synthesizing process performed on voice data through a voice synthesizer of the mixing server according to the embodiment;  
       FIG. 5  illustrates a sequence diagram describing a case where a telephone conference is conducted via an IP network in the IP telephone system according to the embodiment;  
       FIG. 6  illustrates a flow chart describing an operation of IP telephone A in the  FIG. 5  sequence; and  
       FIG. 7  illustrates a flow chart describing an operation of IP telephone C in the  FIG. 5  sequence. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      The embodiments of the present invention are explained in the following, in reference to the above-described drawings.  
       FIG. 1  illustrates a network configuration to which an IP telephone system according to an embodiment of the present invention is applied.  
      The IP telephone system according to the present embodiment includes a plurality of IP telephone apparatuses (hereafter referred to as “IP phone”) A 101 -C 103  connected to an IP network, ENUM server  104 , CA (Call Agent)  105  and mixing server  106 . In the figure, three IP phones A 101 -C 103  are connected to the IP network. The configuration is not limited to this example and may connect four or more IP phones to the IP network.  
      IP phones A 101 -C 103  have a function enabling voice communication with another IP telephone apparatus via the IP network. IP phones A 101 -C 103  also have a function which accesses ENUM server  104  and stores, to ENUM server  104 , a NAPTR resource record (hereafter referred to as “NAPTR record)”, which is described later. More specifically, they have a function which stores a NAPTR record corresponding to a conference number (described later). It is also possible to provide only a specified IP telephone apparatus with the conference number storage function.  
      ENUM server  104  is equipped with a database (DB) that stores a NAPTR record. In response to a query (ENUM query) from one of IP phones A 101 -C 103 , ENUM server  104  transmits, to the IP telephone apparatus that has transmitted the query, the NAPTR record stored in the DB.  
      CA  105  controls voice communication performed between IP telephone apparatuses via the IP network. For example, CA  105  has a function as a SIP (Session Initiation Protocol) server and controls calls between source and destination IP telephone apparatuses.  
      Mixing server  106  performs a synthesizing process on voice data transmitted when voice communication is performed via the IP network. For example, mixing server  106  synthesizes voice data output from IP phones A 101  and B 102 , adjusts the volume and sound quality, and outputs the data to IP phone C 103  when IP phones A 101 , B 102  and C 103  perform a three-party call.  
      In the IP telephone system according to the present embodiment, the DB of ENUM server  104  stores a NAPTR record corresponding to a number (hereafter referred to as “conference number”) which identifies a telephone conference being conducted via the IP network. The telephone conference comprises any type of telephone conversation conducted among a plurality of participants, such as, for example, a meeting, a seminar, a convention, or a session, etc. The NAPTR record corresponding to this conference number includes a URI corresponding to an IP telephone apparatus used by a prospective participant. The prospective participant transmits, to ENUM server  104 , a query for the NAPTR record corresponding to the conference number and obtains identification data for the prospective participant, which enables the person to take part in the telephone conference.  
       FIG. 2  illustrates an example of a NAPTR record corresponding to a conference number stored in the DB of ENUM server  104  in the IP telephone system according to the present embodiment. The figure shows an example where the DB stores a NAPTR record corresponding to a domain name obtained from conference number “1000”.  
      As shown in  FIG. 2 , three URIs “81310001000@tokyo.sip.jp”, “81310002000@tokyo.sip.jp” and “81310003000@tokyo.sip.jp” correspond to domain name “0.0.0.1.3.1.8.e164.arpa”, which is obtained by adding the country code to conference number “1000”. In other words, the example shows that there are three prospective participants for the telephone conference identified by conference number “1000”. Also, the URIs corresponding to IP telephone apparatuses used by these three participants are stored.  
      In the present embodiment, a URI stored in a NAPTR record contains a telephone number of a corresponding IP telephone apparatus. More specifically, the telephone number of the IP telephone apparatus is specified before @. In other words, the example shows that “81310001000”, “81310002000” and “81310003000” are specified as the telephone numbers.  
       FIG. 3  illustrates a functional block diagram for mixing server  106  in the IP telephone system according to the present embodiment.  
      Mixing server  106  is configured with receiver  301 , header processor  302 , voice extractor  303 , voice synthesizer  304  and transmitter  305 .  
      Receiver  301  is connected to the IP network and used to receive communication data for a synthesizing process. In this example, an RTP packet is received as communication data according to an RTP (Real-time Transport Protocol). However, the process is not limited to this example. It is also possible to use other communication packets as communication data according to other protocols such as a UDP (User Datagram Protocol).  
      Header processor  302  interprets data described in the RTP header from the communication data (RTP packet) received by receiver  301  and acknowledges the IP telephone apparatus that has transmitted the RTP packet and the IP telephone apparatus to which the RTP packet has been transmitted. More specifically, header processor  302  acknowledges the telephone numbers of the source and the destination IP telephone apparatuses of the RTP packet by interpreting a port number (input port, output port) described in the RTP header.  
      Voice extractor  303  extracts voice data from the communication data (RTP packet) received by receiver  301 .  
      Voice synthesizer  304  synthesizes the voice data received from voice extractor  303  according to the port number notified by header processor  302 . Voice synthesizer  304  performs a synthesizing process on the voice data when a plurality of voice data are transmitted to a single IP telephone apparatus specified as a destination. In contrast, when voice communication is performed on a one-to-one basis via the IP network, a synthesizing process is not performed on voice data.  
      Transmitter  305  is connected to the IP network and transmits, to the IP telephone apparatus specified as the destination, an RTP packet on which a synthesizing process is performed. In other words, the RTP packet, on which the voice synthesizing process is performed through voice synthesizer  304 , is transmitted to the IP telephone apparatus specified as the destination.  
       FIG. 4  illustrates a diagram describing a synthesizing process performed on voice data through voice synthesizer  304 . This figure illustrates an example where the synthesizing process is performed on voice data transmitted from IP phones A 101 -C 103  shown in  FIG. 1 .  
      In this figure, voice data transmitted from IP phone A 101  are input into input port A. Similarly, voice data transmitted from IP phones B 102  and C 103  are input into input ports B and C respectively. On the other hand, output port A outputs voice data to IP phone A 101 . Similarly, output ports B and C output voice data to IP phones B 102  and C 103  respectively.  
      Voice synthesizer  304  is configured with voice synthesizer units  401 - 403  that synthesize voice data. Voice synthesizer unit  401  synthesizes voice data input into input ports A and B. Voice synthesizer unit  401  is connected to output port C. Accordingly, the voice data synthesized by voice synthesizer unit  401  are transmitted from output port C to IP phone C 103 .  
      Similarly, voice synthesizer unit  402  synthesizes voice data input into input ports B and C and is connected to output port A. Accordingly, the voice data synthesized by voice synthesizer unit  402  is transmitted from output port A to IP phone A 101 .  
      Similarly, voice synthesizer unit  403  synthesizes voice data input into input ports A and C and is connected to output port B. Accordingly, the voice data synthesized by voice synthesizer unit  403  is transmitted from output port B to IP phone B 102 .  
      The following describes a sequence for conducting a telephone conference via the IP network in the IP telephone system having the above-described configuration.  
       FIG. 5  illustrates a sequence diagram describing an example where the telephone conference is conducted via the IP network in the IP telephone system according to the present embodiment. This figure shows an example where three IP phones A 101 -C 103  have a three-party telephone conference via the IP network.  
      URIs “81310001000@tokyo.sip.jp”, “81310002000@tokyo.sip.jp” and “81310003000@tokyo.sip.jp” are assigned to IP phones A 101 , B 102  and C 103  respectively. In other words, telephone numbers “81310001000”, “81310002000” and “81310003000” are assigned to IP phones A 101 , B 102  and C 103  respectively.  
      When the telephone conference is conducted via the IP network, CA  105  controls voice communication performed via the IP network. CA  105  needs to register data for each IP telephone apparatus in order to perform a call control between IP telephone apparatuses. Prior to proceeding to the  FIG. 5  sequence, for example, each IP telephone apparatus transmits the “Register” message to CA  105 , after which CA  105  responds by transmitting the “200 OK” message to each IP telephone apparatus. Through these processes, CA  105  registers data for each IP telephone apparatus beforehand.  
      In the IP telephone system according to the present embodiment, a person in charge of the telephone conference or a person who coordinates the telephone conference (hereafter referred to as “coordinator” for sake of convenience) needs to specify the conference number and to store, in the DB of ENUM server  104 , the NAPTR record corresponding to the conference number. The coordinator also needs to notify prospective participants of conference data such as the conference number and the starting and ending time. Any form, such as telephones, faxes or e-mails, may be used to communicate conference data. The following describes an example where emails are used to communicate conference data. In the following case, the user of IP phone A 101  is the coordinator. The IP phone A 101  user is referred to as “user A”, and the users of IP phones B 102  and C 103  are referred to as “user B” and “user C” respectively.  
      When the telephone conference is conducted via the IP network, the coordinator first stores conference data on ENUM server  104  (ST  501 ). Through this storing process, the conference number as well as prospective participants are specified as conference data. In this example, the DB of ENUM server  104  stores the NAPTR record shown in  FIG. 2 . More specifically, conference number “1000” is specified and the users of IP phones A 101 -C 103  are specified as the prospective participants.  
      When the process of storing the conference data on ENUM server  104  is completed, the coordinator transmits, to the prospective participants, an e-mail message for communicating the conference data (hereafter referred to as “conference data notification email”) (ST  502 ). The conference data notification email contains the conference number, the starting time, and the ending time. The content is not limited to this example and may include other data.  
      When the telephone conference is due to begin, the coordinator inputs conference number “1000” through the numerical keys of IP phone A 101 . The coordinator then instructs IP phone A 101  to transmit, to ENUM server  104 , an ENUM query for a NAPTR record corresponding to the conference number.  
      After receiving the conference number and then the instruction for transmitting the ENUM query, IP phone A 101  transmits the ENUM query to ENUM server  104  (ST  503 ). In this example, IP phone A 101  first converts the coordinator&#39;s input number “1000” into the E.164 number “+81-3-1000” including the country code. Then, “+8131000” is obtained by maintaining + at the beginning and the numbers. Next, non numerical symbols are deleted, and dots are inserted between the numbers, resulting in “8.1.3.1.0.0.0”. Then, the numbers are reversed, and data string “.e164.arpa” is added at the end. As a result, domain name “0.0.0.1.3.1.8.e164.arpa” is obtained. IP phone A  101  then transmits the ENUM query for a NAPTR record corresponding to the domain name.  
      Upon receiving the ENUM query, ENUM server  104  searches for a NAPTR record corresponding to domain name “0.0.0.1.3.1.8.e164.arpa”. ENUM server  104  then transmits, to IP phone A 101 , an ENUM response including the corresponding NAPTR record (ST  504 ). In this example, ENUM server  104  transmits, to IP phone A 101 , the ENUM response including the NAPTR record shown in  FIG. 2 . When the ENUM response is transmitted to IP phone A 101 , the coordinator confirms that users B and C are the prospective participants for the telephone conference specified by conference number “1000” as well as the telephone numbers of IP phones B 102  and C 103 .  
      Upon confirming the telephone numbers of the prospective participants, the coordinator transmits, to IP phone A 101 , an instruction to call one of these telephone numbers. In response to this call instruction, IP phone A 101  transmits, to a selected telephone number, the “INVITE” message via CA  105  (ST  505 ). In this example, the instruction was made to call IP phone B 102 .  
      This example illustrates a case where a call is made to one of these telephone numbers after the coordinator makes the call instruction. However, the process is not limited to this example. It is also possible to make an automatic call to the top telephone number except the telephone number of the source telephone apparatus.  
      Upon receiving the “INVITE” message, IP phone B 102  sounds a ring tone. At the same time, IP phone B 102  transmits, to IP phone A 101 , the “180 Ringing” message via CA  105  (ST  506 ). IP phone A 101  sounds a ring back tone (hereafter referred to as “RBT”) in response to the “180 Ringing” message.  
      When an off-hook operation or other response is detected at IP phone B 102  in response to the ring tone, IP phone B 102  transmits, to IP phone A 101 , the “200 OK” message indicating a connection approval via CA  105  (ST  507 ). Upon receiving the “200 OK” message, IP phone A 101  transmits the “ACK” message to IP phone B 102  (ST  508 ).  
      IP phones A and B obtain the IP address and the input/output port number of mixing server  106  which relays voice data in the sequence starting with the “INVITE” message transmission and ending with the “ACK” message transmission. Upon detecting the “ACK” message, CA  105  notifies mixing server  106  of the port number (input port and output port) used for voice data communication and the IP addresses of IP phones A and B (ST  509 ). The port number is used for voice synthesizer  304  to synthesize voice data. After mixing server  106  is notified of the port number, communication becomes possible between IP phones A 101  and B 102  via mixing server  106 . At this time, voice synthesizer  304  does not perform a synthesizing process, since voice communication is performed on a one-to-one basis.  
      In the present embodiment, a case is explained wherein user C joins the telephone conference later while IP phones A 101  and B 102  are performing voice communication. In order to join the telephone conference later, user C inputs the previously received conference number “1000” through the numerical keys of IP phone C 103  and instructs IP phone C 103  to transmit an ENUM query for a NAPTR record corresponding to the conference number.  
      After receiving the conference number and then the instruction for transmitting the ENUM query, IP phone C 103  transmits the ENUM query to ENUM server  104  (ST  510 ). Upon receiving the ENUM query, ENUM server  104  searches for a NAPTR record corresponding to the ENUM query and transmits, to IP phone C 103 , an ENUM response including the NAPTR record (ST  511 ). The processes performed by IP phone C 103  and by ENUM server  104  are the same as those performed by IP phone A 101  in the above-noted ST  503  and ST  504 , and their descriptions are therefore omitted.  
      When IP phone C 103  receives the ENUM response from ENUM server  104 , user C confirms the telephone numbers of IP phones A 101  and B 102 . In the IP telephone system according to the present embodiment, a participant seeking to join the telephone conference later calls the coordinator for sake of expediency.  
      In this example, the participant who joins the telephone conference later calls the coordinator. However, the process is not limited to this example. It is also possible for the participant to call a user other than the coordinator and to be allowed to join the telephone conference later.  
      Upon confirming the telephone number of the coordinator&#39;s IP phone A 101 , user C instructs IP phone C 103  to call the telephone number. In response to the call instruction, IP phone C 103  transmits, to IP phone A 101 , the “INVITE” message via CA  105  (ST  512 ).  
      Upon receiving the “INVITE” message, IP phone A 101  sounds a ring tone. At this time, IP phone A 101  is communicating with IP phone B 102 , and the ring tone is thus added to the voice during the communication. At the same time, IP phone A 101  transmits the “180 Ringing” message to IP phone C 103  via CA  105  (ST  513 ). IP phone C 103  sounds an RBT in response to the “180 Ringing” message.  
      When the “INVITE” message is received during the communication with another IP telephone apparatus, IP phone A 101  is set to automatically respond to the ring tone. After the automatic response is made, IP phone A 101  transmits, to IP phone C 103 , the “200 OK” message indicating a connection approval via CA  105  (ST  514 ). Upon receiving the “200 OK” message, IP phone C 103  transmits the “ACK” message to IP phone A 101  (ST  515 ).  
      The example shows a case where IP phone A 101  automatically responds to the “INVITE” message. However, the process is not limited to this example. It is also possible to have IP phone A 101  respond in response to a coordinator&#39;s instruction. In this case, the coordinator can determine when letting the delayed participant join the telephone conference. This enables the person to join the telephone conference late without interrupting the telephone conference.  
      In the above-described sequence, every terminal obtains the IP address and the input/output port number of mixing server  106  which relays voice data. Upon detecting the “ACK” message, CA  105  notifies mixing server  106  of the port number (input/output port) used for voice data communication in the same manner as ST  509  (ST  516 ). After mixing server  106  is notified of the port number, communication becomes possible among the three parties, IP phones A 101 , B 102  and C 103 , via mixing server  106 .  
      In other words, voice data output from IP phone B 102  (ST  517 : [B]) and voice data output from IP phone C 103  (ST  518 : [C]) are output to IP phone A 101  as voice data synthesized by mixing server  106  (ST  519 : [B+C]). Voice data output from IP phone C 103  (ST  518 : [C]) and voice data output from IP phone A 101  (ST  520 : [A]) are output to IP phone B 102  as voice data synthesized by mixing server  106  (ST  521 : [A+C]). Further, the voice data output from IP phone B 102  (ST  517 : [B]) and the voice data output from IP phone A 101  (ST  520 : [A]) are output to IP phone C 103  as voice data synthesized by mixing server  106  (ST  522 : [A+B]). Through these processes, communication is performed among the three parties, IP phones A 101 , B 102  and C 103 . The three party communication enables a telephone conference among users A, B and C.  
      The following describes the operations performed between IP phones A 101  and C 103  in the  FIG. 5  sequence. The description of the operation of IP phone B 102  in the  FIG. 5  sequence is omitted.  
       FIG. 6  illustrates a flow chart describing the operation of IP phone A 101  in the  FIG. 5  sequence.  FIG. 7  illustrates a flow chart describing the operation of IP phone C 103  in the  FIG. 5  sequence.  
      When the operation is performed according to the  FIG. 5  sequence, IP phone A 101  first stores conference data on ENUM server  104  according to a coordinator&#39;s instruction (ST  601 ). After completing the process for storing the conference data, IP phone A 101  transmits a conference data notification email to a prospective participant (ST  602 ). In this case, the conference data notification email is transmitted to uses B and C.  
      When the telephone conference is due to begin, IP phone A 101  transmits, to ENUM server  104 , an ENUM query according to the coordinator&#39;s instruction (ST  603 ), after which IP phone A 101  waits for reception of an ENUM response (ST  604 ). When the ENUM response is received, it is determined whether a NAPTR record corresponding to the ENUM query is included in the ENUM response (ST  605 ). When no corresponding NAPTR record is included and no ENUM response is received in ST  604 , IP phone A 101  determines that the communication was unsuccessful and terminates the process.  
      When the NAPTR record corresponding to the ENUM query is included in the ENUM response, IP phone A 101  places a call to the telephone number of another IP telephone apparatus according to the coordinator&#39;s instruction. At this time, IP phone A 101  transmits the “INVITE” message to another IP telephone apparatus (ST  606 ). In this case, the “INVITE” message is transmitted to IP phone B 102 .  
      After transmitting the “INVITE” message, IP phone A 101  determines whether to receive the “200 OK” message (ST  607 ). When the “200 OK” message is received, IP phone A 101  transmits the “ACK” message (ST  608 ). IP phone A 101  continues to monitor for reception of the “200 OK” message until it is received.  
      After IP phone A 101  transmits the “ACK” message, CA  105  notifies mixing server  106  of the port number, after which communication becomes possible between IP phones A 101  and B 102 . Communication starts between IP phones A 101  and B 102  by exchanging voice data (ST  609 ). Such voice data exchange in the communication process is performed via mixing server  106 .  
      While communicating with IP phone B 102 , IP phone A 101  monitors for reception of the “INVITE” message from another IP telephone apparatus (ST  610 ). When the “INVITE” message is received from another IP telephone apparatus, IP phone A 101  adds a ring tone to the voice during the communication. IP phone A 101  also transmits, to the IP telephone that has transmitted the “INVITE” message, the “180 Ringing” message (ST  611 ). In this case, IP phone A 101  receives the “INVITE” message from IP phone C 103  and transmits the “180 Ringing” message to IP phone C 103 .  
      When IP phone A 101  receives the “INVITE” message while communicating with another IP telephone apparatus, IP phone A 101  automatically responds to a ring tone, which sounds in response to the “INVITE” message. After making the automatic response, IP phone A 101  transmits the “200 OK” message via CA  105  (ST  612 ). In this case, IP phone A 101  transmits the “200 OK” message to IP phone C 103 .  
      After transmitting the “200 OK” message, IP phone A 101  determines whether the “ACK” message is received (ST  613 ). When the “ACK” message” is received, voice communication becomes possible among IP phones A 101 , B 102  and C 103  after CA  105  notifies mixing server  106  of the port number. The three-party communication starts among IP phones A 101 , B 102  and C 103  by exchanging voice data (ST  614 ). IP phone A 101  continues to monitor for reception of the “ACK” message until it is received.  
      The voice data exchange in the three-party communication process is performed by mixing server  106  through a synthesizing process. In other words, voice data output from IP phones B 102  and C 103  are output to IP phone A 101  as synthesized voice data. In addition, voice data output from IP phones C 103  and A 101  are output to IP phone B 102  as synthesized voice data. Further, voice data output from IP phones B 102  and A 101  are output to IP phone C 103  as synthesized voice data.  
      As described above, the telephone conference is conducted in the form of a three-party call among users A, B and C. To end the telephone conference, each user performs a terminating process. The telephone conference concludes through the terminating process.  
      On the other hand, when performing operations according to the  FIG. 5  sequence, IP phone C 103  first transmits an ENUM query to ENUM server  104  (ST  701 ). After that, IP phone C 103  waits for an ENUM response (ST  702 ). When the ENUM response is received, it is determined whether a NAPTR record corresponding to the ENUM query is included in the ENUM response (ST  703 ). On the other hand, when no corresponding NAPTR is included and no ENUM response is received in ST  702 , IP phone C 103  determines that the communication was unsuccessful and terminates the process.  
      When the NAPTR record corresponding to the ENUM query is included in the ENUM response, IP phone C 103  places a call to the coordinator&#39;s IP telephone apparatus according to an instruction from user C. At this time, IP phone C 103  transmits the “INVITE” message to the coordinator&#39;s IP telephone apparatus (ST  704 ). In this case, IP phone C 103  transmits the “INVITE” message to IP phone A 101 .  
      After transmitting the “INVITE” message, IP phone C 103  determines whether the “200 OK” message is received (ST  705 ). When the “200 OK” message is received, IP phone C 103  transmits the “ACK” message (ST  706 ). IP phone C 103  continues to monitor for reception of the “200 OK” message until it is received.  
      After transmitting the “ACK” message, voice communication starts among IP phones A 101 , B 102  and C 103  after CA  105  notifies mixing server  106  of the port number (ST  707 ). Such voice data exchange in the communication process is performed by mixing server  106  through a synthesizing process. The voice data synthesizing process is performed in the same manner as ST  614  shown in  FIG. 6 , and its description is therefore omitted.  
      As described above, the telephone conference is conducted by the three-party call among users A, B and C. To end the telephone conference, each user performs a terminating process. The telephone conference concludes through the terminating process.  
      As described above, in the IP telephone system according to the present embodiment, an IP telephone apparatus that attends a telephone conference transmits, to ENUM server  104 , a query for a NAPTR record by specifying a conference number. According to the received NAPTR record, the IP telephone apparatus then obtains identification data for another IP telephone apparatus that is scheduled to attend the telephone conference. This process eliminates the need to individually provide data including another participant&#39;s telephone number. As a result, using the IP telephone system reduces labor required to prepare for the telephone conference, thus making it possible to conduct the telephone conference smoothly.  
      Particularly, in the IP telephone system according to the present embodiment, the DB of ENUM server  104  stores a NAPTR record specifying, in a URI, the telephone number of an IP telephone apparatus. Therefore, an IP telephone apparatus that is scheduled to attend the telephone conference can obtain the telephone number of another IP telephone apparatus that is scheduled to attend the telephone conference. Then, the IP telephone apparatus is able to place a call to another participating IP telephone apparatus using the obtained telephone number.  
      In the IP telephone system according to the present embodiment, mixing server  106  performs a synthesizing process on communication data output from an IP telephone apparatus. Therefore, it is possible to have a real-time telephone conference among three or more IP telephone apparatuses.  
      Particularly, in the IP telephone system according to the present embodiment, mixing server  106  performs a synthesizing process on voice data output from an IP telephone apparatus. Therefore, it is possible to have a real-time telephone conference among three or more IP telephone apparatuses.  
      Further, in the IP telephone system according to the present embodiment, a case is explained wherein mixing server  106  performs a synthesizing process on voice data output from an IP telephone apparatus. However, the process is not limited to this case. It is also possible to have the IP telephone apparatus output image/visual data and to have mixing server  106  perform the synthesizing process on the voice data as well as the image/visual data. In this case, it is possible to have a real-time telephone conference among three or more IP telephone apparatuses, using the voice data as well as the image/visual data.  
      In the IP telephone system according to the present embodiment, a case is explained wherein a three-party telephone conference is conducted by the coordinator placing a call to the telephone number of IP phone B 102 . Communication then starts between IP phones A 101  and B 102 , after which user C joins the telephone conference, performing a three-party telephone conference. However, the process is not limited to this example. It is also possible for the coordinator to call the telephone numbers of IP phone B 102  and IP phone C 103  in this order and to conduct a telephone conference among the three parties after communication becomes possible among users A, B and C. In this case, all participants can speak in the telephone conference from the beginning, and thus more efficient conference operation becomes possible.  
      Further, in the above description, the IP telephone apparatus according to the present invention performs under a SIP as a VoIP protocol. However, the process is not limited to this case. In other words, the configuration may apply to an IP telephone apparatus that performs under H.323 or MGCP (Media Gateway Control Protocol) as the VoIP protocol.  
      The IP telephone described above includes an IP telephone apparatus defined by the government and operated by a telecommunications provider. It also includes an IP telephone provided on a local network or a private network using TCP/IP or other computer network protocols.  
      It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.  
      The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention.  
      This application is based on the Japanese Patent Application No. 2004-211620 filed on Jul. 20, 2004 entire content of which is expressly incorporated by reference herein.