On a POTS network, the only reason for an application to call lineAccept is to inform other applications that it has accepted responsibility to present the call to the user. Similarly, on an ISDN line, the effect of accepting a call is to make other applications aware that some application has accepted responsibility for handling the call.
On an ISDN network, accepting a call also informs the switch that the application will present the call to the user (by alerting the user for example, by ringing or by popping up a dialog box). If the LINEADDRCAPFLAGS_ACCEPTTOALERT bit is set, the application must perform a lineAccept on the call or the call will not ring. If the application fails to call lineAccept quickly enough (the timeout may be as short as three seconds on some ISDN networks), the network will assume that the station is powered off or disconnected and act accordingly, such as by deflecting the call (if Forward on No Answer is activated) or sending a disconnect message to the calling station.
Accepting a call is not the same as answering a call. Answering calls, in POTS, simply means to go offhook. On an ISDN line, it means to tell the switch to place the call in a connected state. Before answering, there is no physical connection for the call between the switch and the destination, though the call is connected from the caller to the switch.
Sometimes a call has already been answered when a new application takes control of it. This can occur, for example, when one application discovers that it is not the highest priority application for a call of a given media mode, and it hands the call off. If the first application has already answered the call, the receiving application takes control of an answered call. It should treat the call normally—that is, as if it had answered the call itself. Another example is when a user instructs an application to operate on an existing call. In this case, the application seizes the call. Again, it should treat the call as if it had answered it.