The lineDevSpecificFeature function enables service providers to provide access to features not offered by other TAPI functions. The meaning of these extensions are device specific, and taking advantage of these extensions requires the application to be fully aware of them.

LONG lineDevSpecificFeature(

HLINE hLine,

DWORD dwFeature,

LPVOID lpParams,

DWORD dwSize




A handle to the line device.


The feature to invoke on the line device. This parameter uses the PHONEBUTTONFUNCTION_ constants.


A pointer to a memory area used to hold a feature-dependent parameter block. The format of this parameter block is device specific and its contents are passed through by TAPI to or from the service provider.


The size of the buffer in bytes.

Return Values

Returns a positive request ID if the function will be completed asynchronously, or a negative error number if an error has occurred. The dwParam2 parameter of the corresponding LINE_REPLY message is zero if the function is successful or it is a negative error number if an error has occurred. Possible return values are:


Additional return values are device specific.


This operation is part of the Extended Telephony services. It provides access to a device-specific feature without defining its meaning. This operation is only available if the application has successfully negotiated a device-specific extension version.

This function provides the application with phone feature-button emulation capabilities. When an application invokes this operation, it specifies the equivalent of a button-press event. This method of invoking features is device dependent, as TAPI does not define their meaning. Note that an application that relies on these device-specific extensions will typically not work with other service provider environments.

Note also that the structure pointed to by lpParams should not contain any pointers because they would not be properly translated (thunked) when running a 16-bit application in a 32-bit version of TAPI and vice versa.

See Also


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