This may help. Modem hang-ups throughout the Internet are very common and can be caused by many different things.
Some users have constant problems, other users never have a disconnect.
The bottom line is that modern modems are pushing the telephone line well beyond what they were originally designed to carry, and thus any small deviation from the ideal causes modems to become confused and drop the line. There is nothing that intentionally disconnects you as long as you are actively using the connection. 20 minutes, IGS will disconnect you automatically when using a PPP connection. The higher the modem speed, the more susceptible to this problem they become. But if your computer is connected to noisy telephone lines, you may see better performance by turning off your modem's error correction.