lineDrop function (tapi.h)

The lineDrop function drops or disconnects the specified call. The application has the option to specify user-user information to be transmitted as part of the call disconnect.


LONG lineDrop(
  HCALL  hCall,
  LPCSTR lpsUserUserInfo,
  DWORD  dwSize



Handle to the call to be dropped. The application must be an owner of the call. The call state of hCall can be any state except idle.


Pointer to a string containing user-user information to be sent to the remote party as part of the call disconnect. This pointer can be left NULL if no user-user information is to be sent. User-user information is only sent if supported by the underlying network (see LINEDEVCAPS). The protocol discriminator field for the user-user information, if required, should appear as the first byte of the buffer pointed to by lpsUserUserInfo, and must be accounted for in dwSize.


Size of the user-user information in lpsUserUserInfo, in bytes. If lpsUserUserInfo is NULL, no user-user information is sent to the calling party and dwSize is ignored.

Return value

Returns a positive request identifier if the function is completed asynchronously, or a negative error number if an error occurs. The dwParam2 parameter of the corresponding LINE_REPLY message is zero if the function succeeds or it is a negative error number if an error occurs. Possible return values are:



When invoking lineDrop, related calls can sometimes be affected as well. For example, dropping a conference call can drop all individual participating calls. LINE_CALLSTATE messages are sent to the application for all calls whose call state is affected. A dropped call typically transitions to the idle state. Invoking lineDrop on a call in the offering state rejects the call. Not all telephone networks provide this capability.

A call in the onholdpending state typically reverts to the connected state. When dropping the consultation call to the third party for a conference call or when removing the third party in a previously established conference call, the provider (and switch) can release the conference bridge and revert the call back to a normal two-party call. If this is the case, hConfCall transitions to the idle state, and the only remaining participating call transitions to the connected state. Some switches automatically "unhold" the other call.

The application has the option to send user-user information at the time of the drop. Even if user-user information can be sent, there is no guarantee that the network will deliver this information to the remote party.

In various bridged or party-line configurations when multiple parties are on the call, lineDrop may not actually clear the call. For example, in a bridged situation, a lineDrop operation may not actually drop the call because the status of other stations on the call may govern; instead, the call may simply be changed to the LINECONNECTEDMODE_INACTIVE mode if it remains connected at other stations.


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

See also

Drop overview

Supplementary Line Service Functions

TAPI 2.2 Reference Overview

Terminate a Session overview