Abstract:
An apparatus and method are disclosed that enable a private branch exchange to determine that a voice mail system has answered a call, without some of the disadvantages in the prior art. The private branch exchange—or, generally-speaking, a data-processing system—infers that the voice mail system has answered a call by timing key events or by noting some of indications that are received from another system, or both. Subsequently, the private branch exchange drops the voice mail system from the call, thereby minimizing the non-enterprise voice mail system&#39;s handling of calls that are originated to an on-premises terminal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to telecommunications in general, and, more particularly, to detecting that a voice mail system has answered a call.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1  depicts a schematic diagram of a telecommunications system in the prior art. Telecommunications system  100  comprises: 
        i. the Public Switched Telephone Network,     ii. private branch exchange  102 ,     iii. telecommunications terminals  103 - 1  and  103 - 2 ,     iv. telecommunications terminals  110 - 1  and  110 - 2 , and     v. telecommunications terminal  111 ; 
 
 all of which are interconnected as shown. 
         
         [0008]     The Public Switched Telephone Network (i.e., element  101 ) is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. In the United States of America, for example, the Public Switched Telephone Network (or “PSTN”) comprises an address space that is defined by ten digits, and, therefore, comprises 10 billion unique addresses or “telephone numbers.” The Public Switched Telephone Networks in other countries are similar.  
         [0009]     The Public Switched Telephone Network provides telecommunications service to telecommunications terminals  110 - 1 ,  110 - 2 , and  111 . Telecommunications terminals  110 - 1 ,  110 - 2 , and  111  can be either wireline terminals or wireless terminals, or a combination of both.  
         [0010]     Private branch exchange  102  is capable of switching incoming calls (e.g., from terminal  111 , etc.) from the Public Switched Telephone Network via one or more transmission lines to terminals  103 - 1  and  103 - 2 . Private branch exchange  102  is also capable of handling outgoing calls from terminals  103 - 1  and  103 - 2  to the Public Switched Telephone Network via one or more transmission lines.  
         [0011]     Private branch exchange  102  is capable of also extending an incoming call (e.g., from terminal  111 , etc.) to a telephone number of an “off-premises” terminal in the Public Switched Telephone Network, in addition to switching the same incoming call to an “on-premises terminal” within the enterprise area (e.g., an office building, etc.) that is served by exchange  102 . Terminals  103 - 1  and  103 - 2  are considered to be on-premises terminals with respect to private branch exchange  102 , while terminals  110 - 1  and  110 - 2  are considered to be off-premises terminals.  
         [0012]     To accomplish the switching of an incoming, enterprise-related call to one of on-premises terminals  103 - 1  and  103 - 2 , and the extending of the call to one of off-premises terminals  110 - 1  and  110 - 2 , private branch exchange  102  maintains a table that correlates the off-premises telephone number to the on-premises, private branch exchange (PBX) extension. Table 1 depicts a table that illustrates the correlation.  
                             TABLE 1                           PBX Extension-to-PSTN Number Database            On-Premises               Telecommunications   Private Branch   Telecommunications       Terminal   Exchange Extension   Network Number               103-1   732-555-0102, x11   201-555-1236       103-2   732-555-0102, x12   908-555-3381       . . .   . . .   . . .                  
 
         [0013]     As an example, a caller at terminal  111  who wishes to reach the PBX user of terminal  103 - 1  dials the PBX number (i.e., 732-555-0102). Private branch exchange  102  receives the incoming call, including the extension number (i.e., x11). Using information stored in memory and similar to what is shown in Table 1, private branch exchange  102  determines that the call is also to be extended to off-premises telephone number 201-555-1236, associated with terminal  110 - 1 . The off-premises number corresponds to the off-premises terminal (e.g., a cellular phone, a home phone, etc.) that belongs to the PBX user of terminal  103 - 1 . The idea behind transmitting the call to both terminal  103 - 1  and  110 - 1  is that if the PBX user is not reachable at his office phone (i.e., terminal  103 - 1 ), then maybe he is reachable at a phone that is outside of the office (i.e., terminal  110 - 1 ).  
         [0014]     One problem that can occur, however, is that the extended call might ultimately be routed to a voice mail system (i.e., a non-enterprise system) that is not associated with private branch exchange  102 . For example, if private branch exchange  102  attempts to reach cellular terminal  110 - 1 , the cellular voice mail system of cellular terminal  110 - 1  might instead answer the call. Consequently, some calls that are made to the PBX user might go to the non-enterprise voice mail system. Meanwhile, other calls that are made to the PBX user might still go to the PBX voice mail system (i.e., the enterprise system), such as when the PBX user disables the call-extending feature when the feature is not needed. It is undesirable for enterprise-related calls to end up in the non-enterprise voice mail system because it burdens the PBX user with having to retrieve voice mail from multiple systems. Also, when accessing the non-enterprise voice mail system, the PBX user is inconvenienced with having to sort out the personal calls that were made directly to the user&#39;s off-premises terminal from the enterprise-related calls that were routed from the PBX.  
         [0015]     What is needed is a technique to minimize the handing of enterprise-related calls by non-enterprise voice mail systems, without some of the disadvantages in the prior art.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention enables a private branch exchange to determine that a voice mail system has answered a call, without some of the disadvantages in the prior art. The private branch exchange—or, generally-speaking, a data-processing system—infers that the voice mail system has answered a call by timing key events or by noting some of indications that are received from another system, or both. Subsequently, the private branch exchange drops the voice mail system from the call, thereby minimizing the non-enterprise voice mail system&#39;s handling of calls that are originated to an on-premises terminal.  
         [0017]     In a first technique, the private branch exchange determines that a voice mail system has answered a call by basing the determination on a time interval, in accordance with the illustrative embodiment of the present invention. As an example, consider an off-premises terminal that is associated with a cellular network. Normally, if the terminal cannot be contacted (e.g., is turned off, etc.), the terminal&#39;s cellular service provider immediately sends the call to the cellular voice mail system. Conversely, if the call is routed into the cellular network and that network is able to reach the terminal, there is typically a several-second delay before the call is answered. Therefore, in accordance with the illustrative embodiment, if the private branch exchange receives an answer indication in less than a pre-determined time, such as four seconds, the private branch exchange assumes that the call has been answered by the cellular voice mail system.  
         [0018]     In a second technique, the private branch exchange determines that a voice mail system has answered a call by basing the determination on message protocols, in accordance with the illustrative embodiment of the present invention. As an example, consider an off-premises terminal that is associated with a cellular network that interacts with its cellular terminals via an end-to-end, Integrated Services Digital Network (ISDN) call model. If the private branch exchange receives an indication that the call has been answered by an endpoint outside the ISDN call model, the private branch exchange assumes that the call has been answered by the cellular voice mail system because cellular voice mail systems often operate in accordance with a non-ISDN protocol.  
         [0019]     The private branch exchange of the illustrative embodiment is also capable of selecting a criterion for determining that a voice mail system has answered the call, wherein the selection of the criterion is based on the called telecommunications terminal. For example, if the called terminal operates in accordance with a first protocol type, then the selected criterion is time-interval-based; if, however, the terminal operates in accordance with a second protocol type, then the selected criterion is message-protocol-based.  
         [0020]     Once the private branch exchange determines that a non-enterprise voice mail system has answered a call made to an on-premises terminal, the exchange handles the call accordingly. The private branch exchange can drop the “leg” of the call that was extended to the off-premises terminal, it can continue to wait for an answer from the on-premises terminal, or it can send the call to the enterprise voice mail system, alone or in combination.  
         [0021]     The illustrative embodiment of the present invention is advantageous over some of the techniques in the prior art. The private branch exchange user, in one technique in the prior art, would have to turn off his cellular voice mail service for all calls, personal and otherwise, to prevent the extended call from going to the cellular voice mail system. In contrast, the illustrative embodiment of the present invention attempts to detect when the cellular voice mail system is answering the extended call, thereby avoiding: (i) the imposition on the user, and (ii) the fallibility of relying on the user to take action.  
         [0022]     The illustrative embodiment of the present invention comprises: transmitting, from a first data-processing system, a call to a second data-processing system; receiving, at the first data-processing system, an indication from the second data processing system that the call has been answered; and ending, at the first data-processing system, the call to the second data processing system based on the time interval between: (i) when the call is transmitted to the second data processing system, and (ii) when the indication is received by the first data processing system that the call has been answered. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  depicts a schematic diagram of a telecommunications system in the prior art.  
         [0024]      FIG. 2  depicts a schematic diagram of a telecommunications system, in accordance with the illustrative embodiment of the present invention.  
         [0025]      FIG. 3  depicts a block diagram of the salient components of private branch exchange  202  in accordance with the illustrative embodiment of the present invention.  
         [0026]      FIG. 4  depicts a flowchart of the salient tasks associated with determining, based on time interval, that a voice mail system has answered a call, in accordance with the illustrative embodiment of the present invention.  
         [0027]      FIGS. 5A and 5B  depict an illustrative example of determining, based on time, whether or not a non-enterprise voice mail system has answered a call.  
         [0028]      FIG. 6  depicts a flowchart of the salient tasks associated with determining, based on message protocols, that a voice mail system has answered a call, in accordance with the illustrative embodiment of the present invention.  
         [0029]      FIG. 7  depicts a flowchart of the salient tasks associated with determining, based on one or more criteria, that a voice mail system has answered a call, in accordance with the illustrative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]      FIG. 2  depicts a schematic diagram of a telecommunications system, in accordance with the illustrative embodiment of the present invention. Telecommunications system  200  comprises: 
        i. the Public Switched Telephone Network,     ii. private branch exchange  202 ,     iii. telecommunications terminals  203 - 1  and  203 - 2 ,     iv. voice mail system  204 ,     v. mobile switching center  208 ,     vi. cellular voice mail system  209 ,     vii. telecommunications terminals  210 - 1  and  210 - 2 , and     viii. telecommunications terminal  211 ; all of which are interconnected as shown.          
         [0039]     The Public Switched Telephone Network (i.e., element  201 ) provides telecommunications service to wireline telecommunications terminals  210 - 1  and  211 , and interconnects multiple telecommunications networks, such as the enterprise network served by private branch exchange  202  and the wireless cellular network served by mobile switching center  208 . The enterprise network served by private branch exchange  202  provides telecommunications service to associated telecommunications terminals, including wireline terminals  203 - 1  and  203 - 2 . The cellular network served by mobile switching center  208  provides telecommunications service to wireless telecommunications terminals, including terminal  210 - 2 .  
         [0040]     It will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention in which some or all of the telecommunications terminals are wireline or some or all are wireless.  
         [0041]     Private branch exchange  202  is a data-processing system, the salient components of which are described below and with respect to  FIG. 3 . Private branch exchange  202  is capable of switching incoming calls (e.g., from terminal  211 , etc.) from the Public Switched Telephone Network via one or more transmission lines to terminals  203 - 1  and  203 - 2 . Private branch exchange  202  is also capable of handling outgoing calls from terminals  203 - 1  and  203 - 2  to the Public Switched Telephone Network via one or more transmission lines (e.g., Integrated Services Digital Network trunks, etc.).  
         [0042]     Private branch exchange  202  is capable of also extending an incoming call (e.g., from terminal  211 , etc.) to a telephone number of an “off-premises” terminal in the Public Switched Telephone Network, in addition to switching the same incoming call to an “on-premises terminal” within the enterprise area (e.g., an office building, etc.) that is served by exchange  202 . Terminals  203 - 1  and  203 - 2  are considered to be on-premises terminals, while terminals  210 - 1  and  210 - 2  are considered to be off-premises terminals with respect to private branch exchange  202 . It will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention in which private branch exchange  202  provides telecommunications service to different numbers of terminals  203  and terminals  210  than those depicted.  
         [0043]     In addition, private branch exchange  202  is capable of performing the tasks described below and with respect to  FIGS. 4 through 7 . Although the illustrative embodiment teaches that a private branch exchange performs the described tasks, it will be clear to those skilled in the art, after reading this disclosure, how to make and use the alternative embodiments of the present invention in which, for example, the tasks are performed by a switch or another type of data-processing system within the Public Switched Telephone Network or another type of network. In other words, the present invention is equally well-suited for implementation in public and private telecommunications systems and in wireline and wireless systems as well.  
         [0044]     Voice mail system  204  is a system for storing voice messages in well-known fashion. Voice mail system  204  is associated with private branch exchange  202  and provides the enterprise voice mail system for one or more users of terminals  203 . It will be clear to those skilled in the art how to make and use voice mail system  204 .  
         [0045]     Mobile switching center  208  is capable of switching incoming calls from the Public Switched Telephone Network via one or more transmission lines to wireless terminal  210 - 2 . Mobile switching center  208  is also capable of handling outgoing calls from terminal  210 - 2  to the Public Switched Telephone Network via one or more transmission lines. Mobile switching center  208  communicates with terminal  210 - 2  via one or more radio base stations, in well-known fashion. It will be clear to those skilled in the art how to make and use mobile switching center  208 .  
         [0046]     Voice mail system  209  is a system for storing voice messages in well-known fashion. Voice mail system  209  is associated with mobile switching center  208  and provides the cellular voice mail system for one or more users of terminals  210 . It will be clear to those skilled in the art how to make and use voice mail system  209 .  
         [0047]      FIG. 3  depicts a block diagram of the salient components of private branch exchange  202  in accordance with the illustrative embodiment of the present invention. Private branch exchange  202  comprises: switching fabric  301 , processor  302 , and memory  303 , interconnected as shown.  
         [0048]     Switching fabric  301  is capable of performing all of the tasks described below and with respect to  FIGS. 4 through 7  under the direction of processor  302 . It will be clear to those skilled in the art, after reading this disclosure, how to make and use switching fabric  301 .  
         [0049]     Processor  302  is a general-purpose processor that is capable of receiving called-related data from switching fabric  301 , of reading data from and writing data to memory  303 , and of executing the tasks described below and with respect to  FIGS. 4 through 7 . In some alternative embodiments of the present invention, processor  302  might be a special-purpose processor. In either case, it will be clear to those skilled in the art, after reading this disclosure, how to make and use processor  302 .  
         [0050]     Memory  303  is a non-volatile random-access memory that stores the instructions and data used by processor  302 . Memory  303  stores the private branch exchange extension and affiliated PSTN telephone number for each PBX user, which are shown in Table 1. In some embodiments, memory  303  also stores the protocol type of the telecommunications terminal (e.g., terminal  210 - 2 , etc.) associated with the PSTN telephone number. Protocol type is described below and with respect to  FIG. 7 . It will be clear to those skilled in the art, after reading this disclosure, how to make and use memory  303 .  
         [0051]      FIG. 4  depicts a flowchart of the salient tasks associated with determining, based on time interval, that a voice mail system has answered a call, in accordance with the illustrative embodiment of the present invention. It will be clear to those skilled in the art which tasks depicted in  FIG. 4  can be performed simultaneously or in a different order than that depicted.  
         [0052]     At task  401 , a first-data processing system receives an incoming call (e.g., from terminal  211 , etc.) in well-known fashion. For the purpose of teaching the tasks described with respect to  FIG. 4 , the first data-processing system is represented by private branch exchange  202 , in accordance with the illustrative embodiment of the present invention.  
         [0053]     Private branch exchange  202  determines, in well-known fashion, that the incoming call is to be extended to both an on-premises terminal (i.e., one of terminals  203 ) and an off-premises terminal (i.e., one of terminals  210 ). For the purpose of teaching the described tasks, it is assumed that private branch exchange  202  selects on-premises terminal  203 - 2  and off-premises terminal  210 - 2  to which to extend the call.  
         [0054]     As part of extending the call to terminal  210 - 2 , at task  402  private branch exchange  202  transmits the call to a second data-processing system, in well-known fashion. For the purpose of teaching the described tasks, the second data-processing system is represented by mobile switching center  208 , in accordance with the illustrative embodiment of the present invention. Private branch exchange  202  remembers, in well-known fashion, the time at which the call is transmitted to mobile switching center  208 .  
         [0055]     At task  403 , private branch exchange  202  receives an indication from mobile switching center  208  that the call has been answered. For example, the call might have been answered by telecommunications terminal  210 - 2  or by cellular voice mail system  209 , in well-known fashion. Private branch exchange  202  remembers, in well-known fashion, the time at which the answer indication is received from mobile switching center  208 .  
         [0056]     At task  404 , private branch exchange  202  determines the time interval between when the call was transmitted to mobile switching center  208  and when the indication was received that the call had been answered. To determine the time interval, private branch exchange  202  calculates the difference between the two times that were remembered as part of tasks  402  and  403 , in accordance with the illustrative embodiment of the present invention. As those who are skilled in the art will appreciate, other methods can be used to determine the time interval.  
         [0057]     At task  405 , private branch exchange  202  checks whether the time interval is no greater than a threshold, T. The value of T can be fixed in software, adjusted by a user (e.g., a system administrator, etc.) of private branch exchange  202 , based on other parameters in telecommunications system  200 , and so forth. For example, in some embodiments, the value can be based on (i) the protocol type of the terminal to which the call is being extended or (ii) the service network of the terminal to which the call is being extended (e.g., the cellular network served by mobile switching center  208 , etc.). It will be clear to those skilled in the art, after reading this disclosure, how to select a suitable value for T.  
         [0058]     If the time interval is no greater than T, task execution proceeds to task  406 . Otherwise, task execution ends.  
         [0059]     At task  406 , private branch exchange  202  ends the call to mobile switching center  208 . In other words, private branch exchange  202  “drops the leg” of the call that exchange  202  extended to off-premises terminal  210 - 2 . Private branch exchange  202  then continues to monitor whether or not on-premises terminal  203 - 2  answers the call.  
         [0060]     At task  407 , private branch exchange  202  optionally forwards the call to a voice mail system (e.g., voice mail system  204 , etc.) that is associated with exchange  202 . For example, private branch exchange  202  might ultimately forward the call to voice mail if on-premises terminal  203 - 2  does not answer the call.  
         [0061]      FIGS. 5A and 5B  depict an illustrative example of determining, based on time, whether or not a non-enterprise voice mail system has answered a call.  FIG. 5A  depicts a scenario in which terminal  210 - 2  answers the call.  FIG. 5B  depicts a scenario in which voice mail system  209  answers the call.  
         [0062]     In  FIG. 5A , terminal  211  transmits call setup message  501  to private branch exchange  202  in well-known fashion. Call setup message  501  corresponds to an incoming call. Upon receiving message  501 , private branch exchange  202  transmits call setup message  502  to mobile switching center  208  for the purpose of extending the call to off-premises terminal  210 - 2 .  
         [0063]     Mobile switching center  208  checks, in well-known fashion, if terminal  210 - 2  is available to receive the call and determines that terminal  210 - 2  is available. As a result, switching center  208  transmits message  503  to send the call to terminal  210 - 2 . Switching center  208  then receives answer message  504  from terminal  210 - 2  and sends answer indication  505  to private branch exchange  202 .  
         [0064]     Private branch exchange  202  determines time interval  511 , which is the difference in time between (i) when exchange  202  transmitted the call to mobile switching center  208 , and (ii) when exchange  202  received the indication that the call had been answered. Exchange  202  compares time interval  511  to threshold  512  and determines that time interval  511  is greater than the value of threshold  512 . Therefore, private branch exchange  202  concludes that terminal  210 - 2  has answered the call and allows terminal  210 - 2  to continue handling the call.  
         [0065]     In  FIG. 5B , terminal  211  transmits call setup message  521  to private branch exchange  202  in well-known fashion. Upon receiving message  521 , private branch exchange  202  transmits call setup message  522  to mobile switching center  208  for the purpose of extending the call to off-premises terminal  210 - 2 .  
         [0066]     Mobile switching center  208  checks, in well-known fashion, if terminal  210 - 2  is available to receive the call and determines that terminal  210 - 2  is unavailable. For example, terminal  210 - 2  might be powered off, might not have returned a response to paging, might not be able to be set up on a traffic channel, and so forth. As a result, switching center  208  transmits message  523  to send the call to cellular voice mail system  209 . Switching center  208  then receives answer message  524  from voice mail system  209  and then sends answer indication  525  to private branch exchange  202 .  
         [0067]     Private branch exchange  202  determines time interval  531 , which is the difference in time between (i) when exchange  202  transmitted the call to mobile switching center  208 , and (ii) when exchange  202  received the indication that the call had been answered. Exchange  202  compares time interval  531  to threshold  512  and determines that time interval  531  is no greater than the value of threshold  512 . Therefore, private branch exchange  202  concludes that terminal  210 - 2  did not answer the call, ends the call to voice mail system  209 , and continues handling the call within its enterprise network.  
         [0068]     Time interval  531  is shorter than time interval  511  because it generally takes longer to page terminal  210 - 2 , set terminal  210 - 2  up on a traffic channel, and receive an indication that terminal  210 - 2 &#39;s user has answered the call than it does to determine that terminal  210 - 2  is unavailable to answer the call. The difference in time intervals is used in accordance with the illustrative embodiment of the present invention to determine if non-enterprise voice mail system  209  answers the call.  
         [0069]      FIG. 6  depicts a flowchart of the salient tasks associated with determining, based on message protocols, that a voice mail system has answered a call, in accordance with the illustrative embodiment of the present invention. It will be clear to those skilled in the art which tasks depicted in  FIG. 6  can be performed simultaneously or in a different order than that depicted.  
         [0070]     At task  601 , a first-data processing system receives an incoming call (e.g., from terminal  211 , etc.) in well-known fashion. For the purpose of teaching the tasks described with respect to  FIG. 6 , the first data-processing system is represented by private branch exchange  202 , in accordance with the illustrative embodiment of the present invention.  
         [0071]     Private branch exchange  202  determines, in well-known fashion, that the incoming call is to be extended to both an on-premises terminal (i.e., one of terminals  203 ) and an off-premises terminal (i.e., one of terminals  210 ). For the purpose of teaching the described tasks, it is assumed that private branch exchange  202  selects on-premises terminal  203 - 2  and off-premises terminal  210 - 2  to which to extend the call.  
         [0072]     As part of extending the call to terminal  210 - 2 , at task  602  private branch exchange  202  transmits the call to a second data-processing system, in well-known fashion. For the purpose of teaching the described tasks, the second data-processing system is represented by mobile switching center  208 , in accordance with the illustrative embodiment of the present invention. Private branch exchange  202  transmits the call to mobile switching center  208  using a first signaling protocol. In accordance with the illustrative embodiment of the present invention, the first signaling protocol used is based on the Integrated Services Digital Network (IDSN) set of protocols, as is well-known in the art. It will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that use other signaling protocols (e.g., Session Initiation Protocol, etc.).  
         [0073]     Optionally, private branch exchange  202  remembers, in well-known fashion, the time at which the call is transmitted to mobile switching center  208 .  
         [0074]     At task  603 , private branch exchange  202  receives an indication from mobile switching center  208  a third data-processing system has answered the call in accordance with a second signaling protocol. For example, telecommunications terminal  210 - 2  or cellular voice mail system  209  might have answered the call, in well-known fashion.  
         [0075]     In accordance with the illustrative embodiment, the indication is received as part of an ISDN CONNECT message and specifies that the off-premises-extended call either is or is not being handled end-to-end as an ISDN call (i.e., a call in accordance with the first protocol). The second protocol used to answer the call can be ISDN-based, proprietary protocol-based, and so forth.  
         [0076]     Optionally, private branch exchange  202  remembers, in well-known fashion, the time at which the answer indication is received from mobile switching center  208 .  
         [0077]     At task  604 , private branch exchange  202  checks whether the second signaling protocol is different than the first signaling protocol. If the second signaling protocol is different than the first signaling protocol (e.g., proprietary-based versus ISDN-based, etc.), then task execution proceeds to task  605 . Otherwise, task execution ends.  
         [0078]     As an example, private branch exchange  202  has transmitted the call to a mobile switching center that is based on the Global System for Mobile Communications (GSM) set of protocols, as is well-known in the art. GSM, along with some other cellular protocols (e.g., Integrated Digital Enhanced Network [iDEN], etc.), is based on an ISDN end-to-end call model. If the returned ISDN CONNECT message indicates that the call has been answered by an ISDN endpoint, it is inferred that a GSM cellular terminal (e.g., terminal  210 - 2 , etc.) has answered the call. If, however, the returned ISDN CONNECT message indicates that the call has not been answered by an ISDN endpoint, it is inferred that a voice mail system, typically proprietary protocol-based, has answered the call.  
         [0079]     At task  605 , private branch exchange  202  ends the call to mobile switching center  208 . In other words, private branch exchange  202  “drops the leg” of the call that exchange  202  extended to off-premises terminal  210 - 2 . Private branch exchange  202  then continues to monitor whether or not on-premises terminal  203 - 2  answers the call.  
         [0080]     At task  606 , private branch exchange  202  optionally forwards the call to a voice mail system (e.g., voice mail system  204 , etc.) that is associated with exchange  202 . For example, private branch exchange  202  might ultimately forward the call to voice mail if on-premises terminal  203 - 2  does not answer the call.  
         [0081]     In some alternative embodiments, the ending of the call to mobile switching center  208  can also be based on the time interval between: (i) when the call is transmitted to switching center  208 , and (ii) when the indication is received by private branch exchange  202  that the call has been answered. For example, if mobile switching center  208  returns a message that indicates that the call has been answered, but does not return an ISDN CONNECT message (i.e., a message with a protocol-related answer indication), then private branch exchange  202  can execute time-based tasks  404  through  407 , described above and with respect to  FIG. 4 .  
         [0082]      FIG. 7  depicts a flowchart of the salient tasks associated with determining, based on one or more criteria, that a voice mail system has answered a call, in accordance with the illustrative embodiment of the present invention. It will be clear to those skilled in the art which tasks depicted in  FIG. 7  can be performed simultaneously or in a different order than that depicted.  
         [0083]     At task  701 , a first-data processing system receives an incoming call (e.g., from terminal  211 , etc.) in well-known fashion. For the purpose of teaching the tasks described with respect to  FIG. 7 , the first data-processing system is represented by private branch exchange  202 , in accordance with the illustrative embodiment of the present invention.  
         [0084]     Private branch exchange  202  determines, in well-known fashion, that the incoming call is to be extended to both an on-premises terminal (i.e., one of terminals  203 ) and an off-premises terminal (i.e., one of terminals  210 ). For the purpose of teaching the described tasks, it is assumed that private branch exchange  202  selects on-premises terminal  203 - 2  and off-premises terminal  210 - 2  to which to extend the call.  
         [0085]     As part of extending the call to terminal  210 - 2 , at task  702  private branch exchange  202  transmits the call to a second data-processing system, in well-known fashion. For the purpose of teaching the described tasks, the second data-processing system is represented by mobile switching center  208 , in accordance with the illustrative embodiment of the present invention. Mobile switching center  208  is serving the telecommunications terminal (i.e., terminal  210 - 2 ) for which the call is intended. Private branch exchange  202  transmits the call to mobile switching center  208  using a first signaling protocol (e.g., ISDN, etc.).  
         [0086]     At task  703 , private branch exchange  202  receives an indication from mobile switching center  208  a third data-processing system has answered the call in accordance with a second signaling protocol. For example, telecommunications terminal  210 - 2  or cellular voice mail system  209  might have answered the call, in well-known fashion.  
         [0087]     In accordance with the illustrative embodiment of the present invention, at task  704  private branch exchange  202  selects, based on the telecommunications terminal for which the call is intended (i.e., terminal  210 - 2 ), one or more criteria with which to make a decision on whether the non-enterprise voice mail system has answered the call. Two examples of criteria selection are presented here.  
         [0088]     In the first example, private branch exchange  202  determines that terminal  210 - 2  is based on the Global System for Mobile Communications (GSM) protocol. GSM, along with some other cellular protocols (e.g., IDEN, etc.), is based on an ISDN end-to-end call model. Therefore, private branch exchange  202  might select criteria that involve basing the call answer decision on message protocols, described above and with respect to the tasks of  FIG. 6 . Private branch exchange  202  determines the protocol type (i.e., GSM) of terminal  210 - 2  by retrieving the protocol type from memory  303 . As those who are skilled in the art will appreciate, there are other ways to determine the protocol type of terminal  210 - 2  (e.g., by querying mobile switching center  208 , etc.).  
         [0089]     In the second example, private branch exchange  202  determines that terminal  210 - 2  is based on the IS-95 Code Division Multiple Access (CDMA) protocol. CDMA, along with some other cellular protocols, is not based on an ISDN end-to-end call model. Therefore, private branch exchange  202  would select one or more criteria that involve basing the call answer decision on something other than message protocols, such as time intervals between relevant events. Time interval-based decision-making is described above and with respect to the tasks of  FIGS. 4, 5A , and  5 B.  
         [0090]     Although the examples described above relate to the protocol type of terminal  210 - 2 , it will be clear to those skilled in the art how to base the selection of one or more decision-making criteria on a different characteristic of terminal  210 - 2  (e.g., its telephone number, etc.).  
         [0091]     At task  705 , private branch exchange  202  checks the criteria selected. If the criteria are message-protocol-based, task execution proceeds to task  706 . Otherwise, task execution proceeds to task  707 .  
         [0092]     At task  706 , private branch exchange  202  executes tasks  604  through  606 .  
         [0093]     At task  707 , private branch exchange  202  executes tasks  404  through  407 .  
         [0094]     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Disclosure, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc.  
         [0095]     Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the disclosure to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Disclosure are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.