It shows TCP timeout which points to some network issue where most likely the keepalives/acks are being missed between cucm and this IP phone. To be able to pinpoint the issue you need to setup a parallel packet capture on the IP phone and cucm to see the direction in which the packets are being dropped.
Cisco TAC could not provide a root cause of this issue.
The phone's Debug display had the following error:
A search for the error returns the following: ReasonForOutOfService=10 -- TCPtimedOut - The TCP connection to the Cisco Unified Communication Manager experienced a timeout error
It was hypothesized that the phone's firmware became corrupt.
I've been able to get the phone to register by bringing the phone back into the office so that it registered on the LAN.