Signed Configuration Files and Moving a Phone From CUCM 8 to CUCME

Communications Manager 8 features signed configuration files by default, however this can cause problems when moving a phone from CUCM 8 to CUCME. The phone will continue to try requesting a signed configuration file (e.g. SEP012345678912.cnf.xml.sgn) instead of an unsigned configuration file (e.g. SEP 012345678912.cnf.xml). As CUCME doesn't by default generate signed configuration files, the phone will fail to download its configuration via TFTP and then fallback to its last known configuration, resulting in a endless loop of registration failure. You can confirm what file the phone is trying to download by enabling debug tftp events.
The solution is to erase the phone's ITL configuration, the process varies between models but for the 7900 series phones it is:

Settings > Security > Trust List > ITL File > **# (to unlock the settings) > Erase

This problem can also bite you when moving a phone between CUCM clusters, Cisco have a support document explaining the fixes here.