4G S8 to be disconnected with 3G shutdown - here is what you can do

I received a nice workaround solution from Samsung support just now: 

Download Samsung Band Selection from google play store, then select Launch Band Selection at bottom, then select WCDMA ALL, then turn phone off and back on. You should be able to make and receive calls.

This worked for me! You just have to refrain from making further changes in the network connection settings because that seems to boot it back to its default settings that don't allow calls to be placed. 

@KNL1990 I wouldn't expect this to work long term once ATT turns off 3G in your area.  Maybe ATT, in the short term, prioritized LTE higher so your phone won't even try 3G.  But if there is no 3G, and no 3G roaming on T-Mobile, I don't see how forcing your phone to 3G will work forever.

I've switched to Google Fi since AT&T service has been so bad dealing with this issue.  If you moving to another carrier take a look at my post on porting your number out to save you self some trouble https://forums.att.com/conversations/wireless-account/port-out-att-number-to-another-carrier/609e18068c74ef3172530143?commentId=623607aa073fde6c48bd418f

I have a Samsumg S8+ (model number G955U1) with a one year prepaid plan that ends in August 2022. After many visits to the AT&T store where I didn't get any encouraging response and when I decided to buy another cell phone, I decided to call to customer service to cancel my plan and ask if I could get a refund from March to August 2022. Right away they gave me the option to send in a new cell phone (S10 e). In less than a week I had my cell phone and hands and now apparently everything is working. Try that! 

KNL1990 - My wife's phone stopped sending/receiving calls March 16, 2022 when a new AT&T billing cycle started (It is now 03/23). Your solution ended up saving us from buying a new phone as AT&T had recommended.  I followed your instructions, but my phone began showing no network on the lock screen. I tried other bands on the App, but ended up using WCDMA ALL, without shutting the phone down again. I did have to pair my phone to my vehicles bluetooth, and sign back in to my WIFI;

@Grundy1700 

remember its a band-aid as some areas still have active 3G towers or you could be roaming.  So it may quit again

Djmdesign, I am inerested in your post as I have a S8 G950U, which shows an ATT logo on boot, already has baseband G950USQU8DUD3 installed, says no software updates available, yet won't show Enhanced LTE or VoLTE options. The ATT list says it should work with a software update, and others have stated that ATT support can "push" that update to your phone, but they won't help me with that software update since I am a Consumer Cellular customer! They won't  even give Consumer Cellular the necessary update.

So how can I fix this issue?

@PaulM78745  get a new SIM card.

https://www.att.com/support/article/wireless/KM1046244/

The AT&T branded model is whitelisted for the network.  

The problem doesn't seem to be the SIM card.  I put the Consumer Cellular supplied SIM card into a friend's TMobile Motorola phone, and was able to make and receive calls over the AT&T network.

The ATT list of compatible phones whitelists this model (and mine does start with an ATT splash screen) but does say a software update might be needed. Apparently they can "push" this update to ATT customers, but of course won't help CS customers using their network! CS say they can't support an ATT phone! So how do I get the update ?

CS say they can't support an ATT phone!

Bovine excrement.

So how do I get the update ?

Make Consumer Cellular treat you like a valued customer.