The Book

https://supportforums.cisco.com/document/19846/cisco-790279057912-ip-phones-are-unusable-or-unresponsive-after-their-firmware

After the default Skinny Call Control Protocol (SCCP) firmware of the 7912G IP phone is changed to Session Initiation Protocol (SIP) firmware and the IP phone is powered on, these issues occur:

• The hold button is red.
• The menu button is green.
• The display window is blank.

This indicates that the phone firmware is corrupted.

If the menu and hold buttons are lit red and green respectively, the phone is in recovery mode. It is possible to return the phone to an operational state.

Complete these steps in order to attempt to recover the phone:

1. Connect a PC to the same network as the phone.
2. Use a network sniffer in order to verify if the phone is operational.

Does the phone transmit any IP packets or request any files from a Trivial FTP (TFTP) server?

Yes The phone is operational. Proceed to step 3.
No The phone is no longer usable and can not be recovered.