Microsoft KB Archive/108305

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INFO: Handling an Existing Call on a Line


The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK) 3.1
  • Microsoft Win32 Application Programming Interface (API)


The Windows Telephony application programming interface (TAPI) provides a way for a TAPI application to handle calls that are initially unknown to the application and/or the Telephony service provider via the LINECALLPARAMFLAGS_IDLE flag when invoking lineMakeCall. In an analog telephone environment, lineGetNewCalls can't provide this functionality for reasons explained below.


A user might manually place a call on a telephone, and then start up his or her computer to send or receive a fax on the same call. A TAPI application has to invoke lineMakeCall to create a call appearance. Normally, an implementation of lineMakeCall checks to see if the line is already in use before it places the call. In an implementation through a modem (such as the sample service provider supplied with the TAPI SDK: ATSP), the service provider does this normally by going off-hook and listening for a dial tone. However, if the application is breaking into a call that already exists on a line, the service provider does not want to listen for the dial tone first--it won't hear it. Additionally, the service provider shouldn't return LINEERR_CALLUNAVAIL because the line is in use.

To handle this situation, TAPI provides the LINECALLPARAMFLAGS_IDLE flag. If the application wants to break into an existing call on the line (not a call known to TAPI but a call that TAPI was not previously aware of), then it will turn off the _IDLE bit. Conversely, if the application wants the provider to start with a "fresh" call (that is, a call with a dial tone), it will set the LINECALLPARAMFLAGS_IDLE bit.

In the case of analog devices, the service provider and TAPI are unaware of the call until lineMakeCall is invoked--lineGetNewCalls cannot be used to get a handle to the call; the call exists, but the software environment is not being synchronized with the "real" environment.

In the digital PBX case, when TSPI_lineOpen is called, the service provider may be able to ask the hardware (phone or switch) about existing calls, and immediately establish these in TAPI by sending a LINE_NEWCALL message and a LINE_CALLSTATE message for each call, showing them as already being in the CONNECTED state; TAPI would give these calls to the highest-priority application with the specified media mode. There would be no need to call lineMakeCall to create a handle to the call, because the provider could do it spontaneously.

Additional query words: 2.00 3.10

Keywords :
Issue type : kbinfo
Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch kbWinSDK310

Last Reviewed: May 12, 2001
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