Our old server just died and we are now trying out Freepbx 15 and Asterisk 18. Almost everything is working now but we seem to have a strange problem with Aastra 6757i phones. They register and work fine for a while but after about 10 minutes Asterisk will delete the contact and the extension will become unreachable. After about an hour the phones will register again and so on. The old server was using Asterisk 11 and Freepbx 12 so I think this behavior is cause by the change form SIP to PJSIP. The phones have the exact same configuration as they did on the old server.
Any settings I should check? Other phones (different models and brands) do not lose registration, only that specific model.
Try setting Max Contacts for the extensions to 3. This should cover up a problem caused by Asterisk seeing the registration from a new address or port. If no luck, we need to see what else e.g. qualify failure is causing the devices to become unreachable. Did the old system have qualify for these phones?
Are the phones on the same LAN as FreePBX? If not, post details about routers/firewalls in the path.
Do the phones show lost registration? Can they make outgoing calls when unreachable?
After many hour diagnosing the problem we found that changing registration time from 600 seconds to 3600 seconds solved the problem. On Asterisk 11 this was never an issue. Phones now stay registered all the time after the change to their configuration.
IMO you should find out what was going wrong. With register expiry set to 3600, if you have to reboot the server or restart Asterisk for any reason, it could be up to one hour before the device re-registers and can receive calls.
On chan_sip, a new registration (different IP and/or port) will overwrite an unexpired old one. With pjsip, if Max Contacts is insufficient to allow another, a new registration is rejected.