Hi Gaven,
This could be caused by any number of issues; including the SCCP IP packets with Cisco Unified Communications Manager (CUCM) being larger in size, when attempting to establish calls, than is required to simply register the phones.
Certainly, start by checking the build of CUCM, and make sure it's not some sort of interim or engineering build; rather than a maintenance release version that has gone through all of Cisco's Quality Assurance testing. Also, check the version of firmware being used on the phone (settings -> Model Information -> Load File).
However, you may also want to set the trace level for CallManager to Detailed, and take a look at why CallManager thinks the phone is resetting.
Please also go to the HTTP server on the phone (http://x.x.x.x/) and take a look at the Console Logs, Core Dumps, etc. for any sign of why the phone rebooted.
Do check the physical connectivity between CUCM and the IP Phone, and make sure all packets will get through without issue. Performing a ping with consecutively larger packets sizes (1024 and above) can sometimes reveal some interesting behaviour over certain types of WAN connections.
Kind regards,
Craig
