An engineer is integrating Unity Connection with Cisco UCM.

Increase the number of voice ports.

If it's a Unity Connection Cluster, ensure that replication is fine and not in split-brain mode.

The phone system to which the phones are registered in Unity Connection has the Default Trap Switch check box enabled.

Add dedicated dial-out ports with the allow trap connections setting selected.

Ensure that you have set up SIP Digest Authentication on the SIP trunk security profile.