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.