This could be a bug (CSCsa67239) which is resolved by upgrading the firmware to 7.2(2). This relates to Cisco IP phone models 7960 and 7940.
Other than that I would suggest removing all records of the phone on callmanager and re-adding it. Verify the URL's in enterprise parameters and make sure DNS is working (you can change the URL's to IP if unsure).
Is this just affecting one phone or multiples?
Please rate if useful.......