Situation:

A Durafon SIP phone set is connected to a Zultys MX-SE IP PBX. The status of the handset and SIP number is active on the Account Settings but the handset cannot make and receive calls. The SIP extension name on the IP PBX was alpha-numeric, “dev2”, and was the same name input as the SIP number on the Account Settings.

Solution:

Set the SIP extension name on the IP PBX as numeric, “200”, instead of alpha-numeric.