Polycom Desk Phones reporting Unregistered periodically...

[TArtusa]

We are using Zoom with a group of older Polycom desk phones. They work fine most of the time, but periodically all phones will report as Unregistered. I have check the firewall and Internet connection and there is no disruption in connectivity.

I was wondering if there a way to allow keep-alives from Zoom through my firewall without opening to many ports or IPs through.