We recently helped a customer with on-premise Cisco CUBE 8300 system where our units were failing to register on the new system. Previously, they used CUCM and our units registered without issue. (our debug logs were showing a SIP Error code: 401 FAILED AUTHORIZATION CODE )
We analyzed and compared the network captures from the customer of the registration process from both systems.
The CUBE was responding to SIP REGISTRATION requests from our unit with a <blank> realm field.
(without this parameter, our unit is unable to generate the hash required to securely continue and complete the registration process)
Once the customer updated the configuration to provide this, our units registered properly.