All the things that Dan and Bob said the phone systems all over america are really...really bad. You have to be plum irrate with them most of the time they will constantly deny all responsibility. More than likely that's the problem.
Don't let the ISP give you the its a Mac crap...I trouble shoot Mac and PC connections everyday and if the world were all Macs ISPs would reduce their tech support calls.
...make sure you modem isn't over heating by say setting it on the monitor or something.
You might check the wall wart to see if its good.
Don't trust the phone company not to put call waiting on the line ...so the friend of the last person that had the number isn't calling it and disconecting you. This happen to me.
Swap out those cables and phone cords.
If you are using a PowerComputing Powerbase and its a busy server the architecture of the machine can cause problems with the serial ports. Could be true for other clones.
