This feature allows the hacker to send arbitrary commands to the modem. One trick is to simply hang-up the modem. Many trojans can contact the hacker as soon as the victim dials up again. Using this trick, the hacker can keep telling the victim to hang up. This essentially stops the user from ever fully connecting to the Internet.
Another trick is when the victim has both a telephone modem and another connection like a DSL line. The hacker can tell the machine to dial "911" and get the police showing up at the victim's door.
Note that the command to hang up a modem is "ATH0".
