You can factory reset a Cisco 7911(G) and 7941/7961(G) at boot time by following the procedure below. Performing a full factory reset will completely wipe the phone. This includes its firmware, leaving the phone in an unusable state if no TFTP server with the necessary files is available, along with a DHCP server that pushes the TFTP server (using DHCP option 150). If you have a Cisco Call Manager, this part should already be set up and the phone will simply download everything from the CCM.
Certain versions of the firmware claim to use another key sequence for resetting the phone. If the above one does not work, try the following sequence:
1 2 3 4 5 6 7 8 9 * 0 #