Applications can communicate with each other by writing to and reading from a field in a specified call's information record, the LINECALLINFO data structure. With lineSetAppSpecific, any owner application can write to the application-specific member called dwAppSpecific. It is uninterpreted by the Telephony API or any of its service providers. This member's usage is entirely controlled by applications.
The member can be read from the LINECALLINFO record returned by lineGetCallInfo. However, lineSetAppSpecific must be used to set the member so that changes become visible to other applications. When this member is changed, all other applications with call handles are sent a LINE_CALLINFO message with an indication that the dwAppSpecific member has changed.