T20P Stops Ringing / Receiving Calls

[rspagan]

Hello,

Occasionally, perhaps once per month, a handful of my T20P phones will stay connected to 3CX, will be able to dial out, but will not be able to receive calls (and the ringer doesn't ring). When dialing the extension, the server cannot locate the user's phone. The phones are not set to DND and they're not on a schedule via the 3CX server. When rebooting the phone, everything comes back to normal.

I have not experienced this with any of my T21P phones.

Any advice would be greatly appreciated!