Those are older phones and it is possible that they will need a two-stage firmware upgrade to talk to current versions of CUCM. If the factory firmware (which is what will be running after a factory reset) is v8.3(3) or earlier, it must be upgraded to a version between that and 8.5(4) before it can then be upgraded to current versions. This is because of a change in the public certificate that Cisco used to sign the firmware. Older phones have the old certificate, and beginning with v8 of CUCM the CUCM server itself will only work with the new certificate. Those interim versions of firmware will interact with both certificates.
So, you will need to set up a router with CME for the purposes of upgrading the firmware on the phones to one of the interim versions. Once that is complete, you can point the phone to your CUCM servers and they should upgrade again.
Here is information on setting up a router with CME for firmware upgrades. Keep in mind that whatever router you use must have the voice licensing installed, but no additional license for CME is required for this temporary purpose:
Upgrade IP Phone Firmware with CCME
Maren
