ATT iPhone 11 Pro problem voicemail not set up

Background:

My i Phone will not receive phone calls. Calls to my iPhone 11 Pro Max will sometimes ring 1/2 ring (sometimes not) and automatically hang up. The following message is played to the caller. “I’m sorry but the person you are calling has a voicemail box that has not been set up yet. Goodbye. “

Troubleshooting and possible solutions:

I have reset the network settings but still get the same voicemail message. I recently updated to iOS 15.2 but the VM issue occurred previous to the update. My voicemail box is set up. DND is off /mute is off /call forwarding is off. Turned Airport mode on and off several times. Pulled SIM card and reseated. I took the phone to ATT store for troubleshooting and the tech said I needed to call ATT advanced tech support and tell them to re-register the phone’s IMEI number. I did this but the ATT tech did not believe this was the issue. Advanced support said they would put in a work order and get back with me in 24-48 hours. It’s been 72 hours and still waiting. No solution as of yet.

I saw some very old post on this subject (2014) but none recently. Anyone else seen this problem?