I have an old post talking about how to factory reset your Cisco IP phone:
However in a lot of cases, after you entering the 3491672850*# sequences twice, you phone looks dead. The right topmost line key will have a orange flash then the screen is dark. In this scenario you need to:
1. Make sure you have DHCP server configured and your phone can get an IP address
2. Make sure you have option 150 ip configured so that the IP phone knows where to get the firmware
3. Finally, you need to change your switchport to "speed 100" manually!!!!! I have four phones recovered from "seems dead" state and it seems to be the trick.
After you make sure you have done the 3 steps above, restart the factory reset steps and you can recover your dead phone!