We’ve had this same sort of behavior on Asterisk 16, and now 20. Essentially Asterisk stops responding to SIP messages, while calls continue as normal. It’s happened to us a few times over the past few years, but since we aren’t Asterisk devs, it’s non-trivial to find out what’s going on.
When this happens we’ve done a core dump instead of merely restarting asterisk. Fairly trivial to do by just sending the process a sigsegv signal:
kill -s SIGSEGV asterisk-process-id
So if it happens to you again, this might prove useful to an Asterisk developer, and (maybe) useful to neophytes if something sticks out like a sore thumb. Dev time is of course a scarce resource, so as Joshua implies, it’s not the most likely thing that anyone will look into it.
I’m by no means an expert, but you can use the script ast_coredumper provided in the script directory to get some somewhat meaningful output from a core dump in a series of txt files listing threads.