The TAPI dynamic-link library (DLL) does not perform probing; that is, it does not send signals on phone lines in order to determine a call's media mode. TAPI does try to deliver the call to an application that can do probing. The way in which TAPI gives calls to applications is determined by several factors, the most important of which are the media mode bits that have been set and the running applications that can (or cannot) handle calls of those media modes.
TAPI's behavior can be divided into two main scenarios: one media mode bit is set, and the UNKNOWN bit is set. These cases are described in the following topics.