Here’s a diagram I used during troubleshooting while I setup these phones about a year ago:
It took some additional configuration to get it to work with AT&T’s system as described here: ATT IP Flex VoIP Nearly Working - #11 by KELSIT
These phones have been using chan_pjsip for nearly a year now and since then I’ve ran updates at least once a month. They started to have this problem after I ran updates, shut everything down, swapped out the server rack, and turned everything back on. I’ve confirmed that the server is connected to the correct interfaces and IP address settings are the same.
The phones are functioning correctly again after converting the extensions from chan_pjsip to chan_sip, but that doesn’t explain why they broke in the first place. I’ve also been told (and have confirmed) that the parking module no longer tells what slot the call was parked in, despite it showing in the logs. I’m not sure if that’s related, but I’d rather fix the pjsip issue first.