I discovered that the fresh install “default” settings for the max duration and max silence settings for the voicemail module are blank.
When a remote caller called in - they had an analog phone plugged into a legacy PBX. When they hung up the phone, they would sometimes initiate a hook-flash on the handset which would cause the remote PBX to put the call on hold or park the call on their side - which was not apparent on the analog handset as it would take a feature code to un-park it.
Neither PBX gave up, they kept the circuit open for 6 hours until my sip trunk provider hung up… I pay by the minute, so this cost a bunch of money!
I put in a bug/feature request to FreePBX to set these voicemail settings in fresh installs to a few minutes so new users on fresh installs don’t learn the hard way… But posting here so others can evaluate if they want to reduce the max voicemail length to something less than hours.