Black Box MD1641A Manuale Utente

After a preset period of time (determined by S-Register S18), the
modem automatically tries to restore the leased line.

The parameters used to determine if a leased line is down (so automatic
dial back can occur), is based on the Series II Modem doing a “retrain”
on the leased line due to an error condition in the transmission. An
error condition is defined as a "hit" on the line (the Carrier gets
interrupted).

The retrain is a "handshake" procedure between the modems to
establish the Carrier again. If the retrain fails, both modems (originate
and answer modems) start their Dial-back timers. The time is
determined by S-Register S19 settings. The S19 default setting is one
minute. During that minute, the originate modem tries to establish the
leased line link. If the leased line is established during that time, the
timer is cleared and everything is back to normal. If the timer expires,
the modems goes to dial-back mode.

The purpose of the timer for the Answer modem is to determine when
it accepts a dial-up call. When both timers have expired and the leased
line has not been established, the dial-back procedure starts. The
number dialed is the one stored in location N9 of the originate
modem's phone number memory. In the preparation for proper 
dial-back operation, enter the proper number in the N9 location using
the commands detailed in Chapter 5. Keep in mind that the number
also can be dialed in V.25bis Command mode.