Is it the radio access network? I know my phone can switch to VoIP is the radio signal is insufficient.Yeh waiting on a job offer. What a time to go down. Couldn't access VM. Apparently Voice call services down the last 3 hours per 3's X account. Otherwise I rarely use voice calls.
So sticking it aeroplane mode and then turning on the wifi might fix your issue.
Its really a question of if the issue within the RAN or HSS.