At what moments can mgetty exert control over the line? Let's
consider mgetty's life span: at some time (e.g. system boot, init
assumes the specified run level and starts mgetty) mgetty starts and
(simplified) checks for the lock file on the device it has to
control. If there's no lock file, it initializes the modem and wait's
for an event on the line. If a character arrives, mgetty does not read
it but first checks the lock file. 

* If a lock file is present, mgetty knows some{body,thing} wants to
dial out and periodically checks for the existence of the lock
file. After the lock file is gone, the device is free and mgetty
simply exits. It'll get restarted by init, so the modem line will
get reinitialized. 
* If no lock file is present, the modem most probably sent a RING, so
we go check for the expected word (= RING) and send our
answer_chat. If it's a fax call (and the modem can receive faxes and
we allow receiving them) we get the pages, store 'em away and exit,
thus restarting mgetty, see above. If it's a data call (I ignore
DISTINGIVE_RING and Caller_ID features for now), we prompt for the
login and wait for an answer. After the answer is read, it's checked
against the login.config file and the appropriate program get's exec'd
by mgetty, which means, there's *no* mgetty for that line. Only after
the termination of the connection, when the other process(es) exit(s),
mgetty will be restarted by init.