Usually this happens because the modem is set to ignore a DTR drop and still has a data connection to the other side. If you use a voice modem, it could be stuck in voice mode. In these situations, the normal init-chat will time out, because the modem won't send the proper responses back. Some modems forget parts of their settings when going from data to fax mode and back during modem initialization. For those modems, you can use this command to set up some AT commands that are executed after all other fax and voice initialization has been done.
Some modems have the nasty tendency to crash silently.
If not, mgetty will restart itself and do a full modem reset. Default is 1. The MSN called will be compared the list, and the first match is used for the distinctive RING number. The list is searched from left to right. This is known to work with ELSA and ZyXEL ISDN terminal adaptors.