What image is loaded on the 7920 currently?
What is the name set for the default 7920 image?
Is the 7920 configured as a 7920 or 7960?
What's the CallManager revision?
After it reboots, check the firmware rev then as well.
Could possibly be using the os7920.txt file as well in tftp path. Ensure the contents of this file match what is listed as the 7920 device default image.