Comcast Outbound Landline Calls Drop on Our End After About 20 Seconds

hear them.  This is on every call.  Incoming calls are no problem.

Comcast has been out and suggested we plug the base unit of our wireless phones directly into the modem.  We have done that and it has not fixed it.  We have tried a different phone and it did not fixed it.  We have restarted the router and that did not fix it.  They are now blaming it on the fact that we have a non Comcast modem, which I do not believe is the problem.

Has anyone had a similar problem?