Recurrent PC Lock Up

We have been bothered by a persistent PC Lock Up when doing work in Office Products. Much troubleshooting and hair pulling later we have discovered that the PC is hooked to a multiline phone and each time the phone rings, either to an answering machine or to an open line. It Locks up. We have removed the phone connection from the modem and the problem has ceased. Do you have any suggestions on troubleshooting the modem or compatibility issues that coud help us solve this problem? I would like to add that we have not yet tried another modem, and plan to do so. If that helps, I’ll let you know. But this is weird…
Thanks
kyfreds@yahoo.com