The \ModemControl plug-in module expects four parameters as follows:
Function identifies the calling condition for the plug-in:
0 about to begin a dial attempt for an outgoing call
1 completion of a successful outgoing call
2 failure of an outgoing call
3 modem is going idle after a call or on exit from a failed mode
4 modem is being set into a failed mode
Index is the index of this modem device in the list of system modem devices (as returned by the ModemList function).
Tag is the name of the tag that originated the call (functions 0, 1, and 2 only).
UserData is the opaque user data supplied as a parameter to the MakeCall() method (functions 0, 1, and 2 only).