modem

The modem command tells Autolog what type of modem you're using and therefore how to translate commands such as dial and hangup into the specific commands used by your modem to perform these actions.
 
modem at

tells Autolog that you're using a modem that uses the AT command set.

You must link to your modem's port before using the modem command. It is also best to make adjustments to the communications port's settings (such as the baud rate) before using the modem command.