WinNT 4.0 & Remote Access

We have a small network of about 20 PCs. Our server uses Windows NT 4.0. The other PCs are a mixture of Win95 and Win98. We have a few portables used by the salesmen who dial in to the server, with dial-up networking, and connect to the network where they can check their mail and update the database. The server has a U.S. Robotics 56K Faxmodem. Our problem is after 7 successful Remote Access connections, the modem on the server stops answering the call. The only fix I have found is to reboot the server and it seems to clear it up until we get another 7 calls. I’ve tried to look at different settings with no success. If anyone could give me some “words of wisdom”, I’d really appreciate it. Thanks!