phoneInitializeExA function

The phoneInitializeEx function initializes the application's use of TAPI for subsequent use of the phone abstraction. It registers the application's specified notification mechanism and returns the number of phone devices available to the application. A phone device is any device that provides an implementation for the phone-prefixed functions in the Telephony API.


LONG phoneInitializeExA(
  LPHPHONEAPP               lphPhoneApp,
  HINSTANCE                 hInstance,
  PHONECALLBACK             lpfnCallback,
  LPCSTR                    lpszFriendlyAppName,
  LPDWORD                   lpdwNumDevs,
  LPDWORD                   lpdwAPIVersion,



Pointer to a location that is filled with the application's usage handle for TAPI.


Instance handle of the client application or DLL. The application or DLL can pass NULL for this parameter, in which case TAPI uses the module handle of the root executable of the process.


Address of a callback function that is invoked to determine status and events on the line device, addresses, or calls, when the application is using the "hidden window" method of event notification (for more information see phoneCallbackFunc). This parameter is ignored and should be set to NULL when the application chooses to use the "event handle" or "completion port" event notification mechanisms.


Pointer to a null-terminated string that contains only displayable characters. If this parameter is not NULL, it contains an application-supplied name for the application. This name is provided in the PHONESTATUS structure to indicate, in a user-friendly way, which application has ownership of the phone device. If lpszFriendlyAppName is NULL, the application's module filename is used instead (as returned by the function GetModuleFileName).


Pointer to a DWORD. Upon successful completion of this request, this location is filled with the number of phone devices available to the application.


Pointer to a DWORD. The application must initialize this DWORD, before calling this function, to the highest API version it is designed to support (for example, the same value it would pass into dwAPIHighVersion parameter of phoneNegotiateAPIVersion). Artificially high values must not be used; the value must be accurately set. TAPI translates any newer messages or structures into values or formats supported by the application's version. Upon successful completion of this request, this location is filled with the highest API version supported by TAPI, thereby allowing the application to detect and adapt to having been installed on a system with an older version of TAPI.


Pointer to a structure of type PHONEINITIALIZEEXPARAMS containing additional parameters used to establish the association between the application and TAPI (specifically, the application's selected event notification mechanism and associated parameters).

Return Value

Returns zero if the request succeeds or a negative error number if an error occurs. Possible return values are:



Applications must select one of three mechanisms by which TAPI notifies the application of telephony events: Hidden Window, Event Handle, or Completion Port.

  • The Hidden Window mechanism is selected by specifying PHONEINITIALIZEEXOPTION_USEHIDDENWINDOW in the dwOptions member in the PHONEINITIALIZEEXPARAMS structure. In this mechanism (which is the only mechanism available to TAPI version 1.x applications), TAPI creates a window in the context of the application during the phoneInitializeEx function, and subclasses the window so that all messages posted to it are handled by a WNDPROC in TAPI itself. When TAPI has a message to deliver to the application, TAPI posts a message to the hidden window. When the message is received (which can happen only when the application calls the Windows GetMessage function), Windows switches the process context to that of the application and invokes the WNDPROC in TAPI. TAPI then delivers the message to the application by calling the phoneCallbackFunc, a pointer to which the application provided as a parameter in its call to phoneInitializeEx (or phoneInitialize, for TAPI version 1.3 and 1.4 applications). This mechanism requires the application to have a message queue (which is not desirable for service processes) and to service that queue regularly to avoid delaying processing of telephony events. The hidden window is destroyed by TAPI during the phoneShutdown function.
  • The Event Handle mechanism is selected by specifying PHONEINITIALIZEEXOPTION_USEEVENT in the dwOptions member in the PHONEINITIALIZEEXPARAMS structure. In this mechanism, TAPI creates an event object on behalf of the application, and returns a handle to the object in the hEvent member in PHONEINITIALIZEEXPARAMS. The application must not manipulate this event in any manner (for example, must not call SetEvent, ResetEvent, CloseHandle, and so on) or undefined behavior results; the application can only wait on this event using functions such as WaitForSingleObject or MsgWaitForMultipleObjects. TAPI signals this event whenever a telephony event notification is pending for the application; the application must call phoneGetMessage to fetch the contents of the message. The event is reset by TAPI when no events are pending. The event handle is closed and the event object destroyed by TAPI during the phoneShutdown function. The application is not required to wait on the event handle that is created; the application could choose instead to call phoneGetMessage and have it block waiting for a message to be queued.
  • The Completion Port mechanism is selected by specifying PHONEINITIALIZEEXOPTION_USECOMPLETION PORT in the dwOptions member in the PHONEINITIALIZEEXPARAMS structure. In this mechanism, whenever a telephony event needs to be sent to the application, TAPI sends it to the application using PostQueuedCompletionStatus to the completion port that the application specified in the hCompletionPort member in PHONEINITIALIZEEXPARAMS, tagged with the completion key that the application specified in the dwCompletionKey member in PHONEINITIALIZEEXPARAMS. The application must have previously created the completion port using CreateIoCompletionPort. The applications retrieves events using GetQueuedCompletionStatus. Upon return from GetQueuedCompletionStatus, the application has the specified dwCompletionKey written to the DWORD pointed to by the lpCompletionKey parameter, and a pointer to a PHONEMESSAGE structure returned to the location pointed to by lpOverlapped. After the application has processed the event, the application must call LocalFree to release the memory used to contain the PHONEMESSAGE structure. Because the application created the completion port (thereby allowing it to be shared for other purposes), the application must close it; the application must not close the completion port until after calling phoneShutdown.
When a multithreaded application is using the Event Handle mechanism and more than one thread is waiting on the handle, or the Completion Port notification mechanism and more than one thread is waiting on the port, it is possible for telephony events to be processed out of sequence. This is not due to the sequence of delivery of events from TAPI, but would be caused by the time slicing of threads or the execution of threads on separate processors.

If PHONEERR_REINIT is returned and TAPI reinitialization has been requested (for example, as a result of adding or removing a telephony service provider), then phoneInitializeEx requests are rejected with this error until the last application shuts down its usage of the API (using phoneShutdown). At that time, the new configuration becomes effective and applications are once again permitted to call phoneInitializeEx.

If the PHONEERR_INVALPARAM error value is returned, the specified hInstance parameter is invalid.

The application can refer to individual phone devices by using phone device identifiers that range from zero to dwNumDevs minus one. An application should not assume that these phone devices are capable of any particular TAPI function without first querying their device capabilities by phoneGetDevCaps.


Target Platform Windows
Header tapi.h
Library Tapi32.lib
DLL Tapi32.dll

See Also




Supplementary Phone Service Functions

TAPI 2.2 Reference Overview