We are having a similar issue with wifi calling over 2degrees broadband and it appears to have started for us with update to iOS 12.x as prior to that we had no issues since taking up the service.

Symptoms include outbound SMS will get to 3/4 send then fail, outbound phone calls will attempt to dial out then fail after small time delay of silence, workaround is to turn phone into airplane mode and back again (sometimes twice) then it will work fine for a while then revert again. Called 2degrees and they advise no idea on root cause, we are going to screen record the issue and send them more data but interested in any other case #'s or solutions others may have had.

Thanks