For what it's worth, we've been dealing with a similar issue, using a BYOD IP PBX system.

System has been working near flawlessly for several months, then starting around March 12, we started getting periodic drops in connectivity.

This results in incoming calls going to failover. And outbound calls fail, telling us basically that we do not have a connected trunk.

Changing to SIP2 seems to have solved the issue for several locations, but not our busiest installation.