Question

A new Cisco IP Phone 8865 configured for SIP is failing to register with CUCM. Packet captures show the phone sends a REGISTER, CUCM responds with a 401 Unauthorized containing a WWW-Authenticate header, and the phone resends the REGISTER with an Authorization header. CUCM then replies with another 401 and the cycle repeats indefinitely. Other phones on the same subnet using the same device pool register successfully. What is the most likely cause?

Accepted Answer

The Digest Credentials configured on the End User associated with the phone do not match the digest password entered on the phone's Phone Configuration page

Answer

The phone's firmware load is incompatible with the CUCM version, blocking the TLS handshake

Answer

The SIP Profile applied to the phone has the wrong SIP Trunk Security Profile assigned

Answer

The phone is receiving the wrong VLAN from the switch and cannot reach the CUCM subscriber