T46S Phones Continue to Register and then become Unregistered

The plague of the T46S has landed on us.  Several months ago we deployed an office using Yealink T46S phones.  The phones would become unregistered.  Reboots initially indicated that the problem was cleared when the phone re-registered.  We thought the carrier was the issue and had them change out the router.  Did not clear the issue.  Changed PoE switches, UPS units and opened a Sangoma paid ticket to resolve the issue.  Could not get resolve so we pulled the phones and had the customer go a different direction as we could not get resolve.

Yealink T46S phones unregistered again, round two.  Took the same phones to a new install, factory defaulted them and placed into the same FreePBX hosted pbx.  Now in a different location, same issue.

Upgrade firmware, insure the power settings were turned off, upgraded the PoE switch to Ubiquiti switch so that we view traffic and still the phones randomly unregister.

The web is littered with similar issues on this specific phone, so the question is, what is the resolve?