Need some help please...

First, I'm not a phone guy so please excuse my lack of knowledge - but I'm trying to help a customer out. They have a 3Com IP phone system with a dozen or so Centrex lines coming in. Occasionally they report echo problems on BOTH internal and external calls. The problem appears to be random and sporatic.

As they experience this on internal calls as well as external, my initial thought is that it is an internal and/or 3Com issue. Does that sound right? Could it be the telephone company? Where should we start???

Any and all help appreciated!