I've a small IP VoIP testing lab (2811 & 3550) to which I've attached 7 7960s. One is a "good" phone, and the others have been sent in by end-users with tags indicating "will not register". The phones all registered on the test system, but when I make calls from one phone to the other six, sometimes the receiving phone rings and sometimes it does not. This changes changes randomly (as far as I can tell) and does not seem to be related to which phone is used to call from. It feels like a problem with the switch or router, rather than the phones. Router config attached.