Need way more information to help you. What version of IVM are you running?
If prior to 3.0; if you are using Vontage or some other VOIP device, it should feed into the modems like a normal phone line and really has nothing to do with VOIP itself. Probably a modem compatibility issue. 
If you are using 3.0; you can receive VOIP directly from your broadband service. Make sure your firewall has all the right ports directed to your IVM box, and that you don't have any other VOIP software installed beside IVM that could be competing for the VOIP services. 