I can confirm that my 502 also became unregistered on the vPanel overnight. VPanel shows no devices registered yet the 502 thinks it is registered. Also, when calling my number it did NOT go to my failover. It rang once then a few secs of silence then a female recorded voice said "Bye" and I was disconnected.

My 502 is connected directly to cable modem.

A reboot of the 502 did fix the problem.