Can't make or receive phone calls on an AT&T S20 Ultra

[userRLofioEJFo]

I have a two year old, AT&T cobranded Samsung Galaxy S20 Ultra, which as of today (4/30/22) can no longer make or receive phone calls:

• When I try to dial out, the phone immediately goes to "call ended", as if I hung up
• Incoming calls don't go through at all.  Moreover, the call hears that my mailbox isn't set up.  Normally, I used Google Voice for voicemail, and that had been working fine until today
• All other features, including data over wireless and SMS, work perfectly fine.  That is, these work fine when WiFi and Bluetooth are disabled, so it's definitely connecting over the AT&T network

Troubleshooting thus far:

• Tried rebooting the phone, going into safe mode, refreshing the connection from att.com/support
• The phone and all apps are running the latest versions available - no updates pending
• I removed the Google Voice app in case it was causing any issues
• I tried using the MMI codes (e.g., ##61#) to disable forwarding to Google Voice, but get a connection error (801) when sending the code
• Spoke with customer service on the phone and went through all their troubleshooting steps, including resetting network settings.  He eventually suggested I replace the SIM
• Went to at AT&T store and bought a new SIM, which didn't help.  They did mention they have had other customers in this week with similar problems with other Samsung phones.  However, I can't seem to find anything by Googling around

Current status:

• SMS and data work perfectly fine over the AT&T network (when disconnected from WiFi and Bluetooth) but still cannot make or receive calls
• Tried everything I can think of short of a full factory reset
• My wife's phone on the same AT&T account, and my work-provided AT&T phone, all work normally from this.  I've never had any connection issues in the past in this location
• Phone shows 5 bars for signal strength, and connected to a 5Ge network
• From the SIM card status menu, Mobile Network State = Connected, Service State = In Service, Mobile voice network type = LTE
• Phone = AT&T co-branded, network locked, SM-G988U

The guys at the AT&T store suggested waiting for AT&T or Samsung to push out a fix for the issue, since other people have had the same problem.  They said when they contacted the AT&T home office, the person they spoke with said they were aware of the issue.  However, I haven't been able to find anything online about it - you'd figure if it was a fairly widespread issue, there'd be forum posts or news about it somewhere.

Any suggestions on next steps?  Full factory reset isn't really an option - I'd sooner buy a new phone and make sure I've transferred everything successfully before wiping my current one.

[Cass94]

It sounds like you've tried most steps I would try but sometimes with things like this doing certain steps in a certain order(even if already tried separately) can be the trick...that said I can walk you through another "way" of resetting some things that could fix it

[userRLofioEJFo]

Any other suggestions would be greatly appreciated.  My only other options now are to wait and home AT&T/Samsung fix it, or buy a new phone.

[Cass94]

Okay again may not work but let's try

[Cass94]

This will involve things I'm sure you already tried however sometimes it is necessary to do these in a specific order and you get a different result than if you did the same resets out of order

[Cass94]

So there's a short method we can try first then a longer if that doesn't fix it or just skip to longer I would try the simple first just cuz we don't need to reset more than necessary

[Cass94]

Let me know when your ready to begin but I do believe the issue is with how the hierarchical file system works and that what's happening is that when you clear out "reset" the files they are being loaded back in instantly with the same bug occurring because the "parent" files to the ones you delete are not being reset thus you can reset a million times but it won't change anything because the files that instruct replacement are corrupt

[userRLofioEJFo]

rdy now, thanks

[Cass94]

Okay so like I said hopefully the trick will be simply resetting some things WHILE restricting the phone from finishing the reset until all files are purged

[Cass94]

So how familiar are you with these phones? I want to stress the only difference in what I tell you and what you might have already tried will be the order you do it in and combining more than one reset before you allow phone to re load files