In the event that a SPA50xG IP phone experiences an interruption of power during a firmware upgrade, the phone's firmware images could be rendered unusable. If this occurs, the phone will display a message "SOS phone in recovery mode".
The phone will automatically attempt to acquire a valid image from the "last upgrade URL" that it knows about.
You can manually recover a SPA50xG phone from this state using the attached recovery utility. You can use the utility regardless of the firmware version of the phone.
Using the recovery utility
When the phone is trying to access the “last upgrade URL”, it will not be able to handle a recovery request from the PC. Once the phone gives up, you will be able to successfully use the recovery utility:
Make sure the default TFTP port (69) is not occupied on the computer. For example if you have a TFTP server such as Pumpkin or SolarWinds running, shut the TFTP service down.
You might have to disable the firewall or other security measures on the PC to allow TFTP to function properly.
Run the recovery utility and enter the phone’s SN when prompted. You can use “FFFFFFFFFFFF” (12 ‘F’s) instead of the phone's serial number.
The recovery utility should find the phone in recovery mode.
Follow the instructions on screen to start the recovery process.
Once the phone has recovered and rebooted, you can upgrade to the desired version of firmware.