If you want to prevent mgetty from accepting calls, you can do so by
creating a file called /etc/nologin.<device>
(e.g. /etc/nologin.tty2a). If mgetty detects an incoming call, and
sees this file, it will NOT answer the phone. This way the caller
does not lose money for a useless call when logins are not allowed
anyway. (You can change the filename that is checked by changing
NOLOGIN_FILE in policy.h)
This can be used for quite sophisticated scheduling purposes - imagine a
modem attached to a line used for voice during the daytime. So, you want
the modem only to answer the phone during 9 pm and 7 am. What you do is to
have cron create a /etc/nologin.device file at 7 am (so
the modem won’t answer the call during the day) and remove it at 9 pm (so
the modem can answer the phone at night).
Naturally, this can be easily extended - e.g., answer the phone only on weekends (similar cron job), don’t answer the phone if there are less than 5 Mbyte free on disk (have a process check for free disk space regularily and create /etc/nologin.* file(s) accordingly), …