From the phone's Product Manager (SC)
The SPA501G uses an LED sequence, to indicate that the phone is upgrading. LEDs are used because this phone does not have a display.
During the upgrade process, all the Line Keys light up and blink in a special sequence to indicate that a firmware upgrade is in progress. The LED sequence is similar to the other SPA500 Series IP Phones
During the upgrade period, indicated by flashing LEDs, we highly recommend that the phone is not power cycled or disconnected from the network.
The phone has intelligence to ensure that the newly downloaded firmware image is complete and undamaged before making it the phone's primary image.
The firmware update methodology is robust. In the event that the phone's primary and secondary images are corrupt, the phone will enter an SOS-mode where the LED's flash s-o-s.
A Windows recovery .exe image is available for recovering the phone from SOS-mode.
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