Strange - Intermittent issues with inbound SIP calls

We have two Asterisk installations on FreePBX 13 that are both configured virtually identically and both having a sporadic issue with inbound SIP calls.

I can make 100 calls to the same inbound telephone # and some random number of them will fail with a message "NOTICE… chan_sip.c … handle_request_invite: Call from ‘voip-pbx’ (IP of SIP) to extension ‘+18005551212’ rejected because extension not found in context ‘internal’.

Some interesting information: Originally, the FreePBX/trunks/SIP/peer details showed context=‘internal’. However, watching the inbound calls, I never saw that context execute on either the working calls or the failed calls.

I switched that context=“internal” to “pstn-3164-us” to match the definitions in “sip_custom.conf”. Now when the call fails,the error reports context ‘pstn–e164-us’.

Both contexts should be valid and as they work for about 90% of the calls, this is a bit hard to figure out.

Something is obviously causing a small percent of the calls to fail, but I don’t know where to look.

The “full” log does not give any clues. Setting the debug level to 3 doesn’t either.

I have tried to find clues in the CEL tables, but nothing there either.

Any assistance is appreciated.