Modem installation issue

I recently helped a friend install a modem on his Optiplex GXa system (PII-300, mid-size chasis) and ran into problems. The modem is a USR 56k fax modem (3cp5699a) and I downloaded the latest drivers for Win 98. He's all updated in Windows currently. It connects properly, but will not disconnect when connection is terminated on the computer (you have to unplug the phone jack from the wall). It doesn't seem to actually be connected since there's no chatter on the phone line. Any ideas on what the problem could be? I've tried reinstalling the modem several times.