This is a common problem with phones that have an older firmware trying to register with CUCM that contains a newer device default version. A conflict comes into play and the phone is ultimately rejected. You would first need to verify the phone load on your phone your having trouble with, then verify the device default firmware for the phone model by going to Device > Device Settings > Device Defaults. Scroll down to the phone model and verify the phone load. If this phone load is different from whats on the phone your having problems with you then will need to copy the phone load in Device Defaults and paste it within the phone configuration page in CUCM, save the changes and restart your TFTP Service on the Subscriber that phone is registered with. You should see the phone go through its process in downloading its new firmware which may take longer from a standard phone restart. Keep in mind if your working on a production environment, that you open a change request (If your follow ITIL standards) and execute this after hours.
