It has an initial conversation with the modem, so that it can be set in the correct mode, and then waits for the modem to detect an incoming call. It then spawns getty(8) to login the user, after making an alteration in the file /etc/utmp, so that getty thinks it was spawned by init(8).
While the user remains logged on, modemB will periodically run ps on the line, so that it can record what the user is doing. This facility can be turned off (see WATCHIT) option, if not required.
If, at any point prior to the incoming call, modem detects an outgoing call, then it exits. Just before the exit, it attempts to set the line so that it can be used by outgoing callers.
If modem detects a LOCK file when it is invoked, then it will wait until this file is removed before proceeding.
modem also enforces an idle timeout, so that it the line appears to be idle for too long, the user is logged out. This may cause problems with kermit, which doesn't update the times on the device, but uses /dev/tty instead.
The optional speed argument can be used to force modem to talk to the modem at the specified speed, otherwise it will talk at 1200 baud.
The tty name should be of the form tty44.
44:2:respawn:/usr/lib/modem tty44
1a:234:respawn:/usr/lib/modem 2400 tty1a
There are still some unexplained diagnostics in my log file, mainly about write(1) calls failing to write characters.
You will have to modify some source code if your modem and/or system is different from mine (speed configuration and lock files).