ATT said my S6 will no longer work and just deactivated it after sending me another phone. Is there any way I can still make it work?

This ⬆️

You cannot use the S6 anymore. You will have to replace it with a phone that is on a T and T whitelist

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/idpassets/images/support/wireless/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf&ved=2ahUKEwj2m6-ek6DyAhUMZd8KHYxaAAUQFnoECAQQAg&usg=AOvVaw1C5znQishzVLbCB1H7kNIA

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/idpassets/images/support/wireless/Devices-Working-on-ATT-Network.pdf&ved=2ahUKEwjH3_fZkuDwAhUKGFkFHbHnBkEQFnoECBcQAQ&usg=AOvVaw3j95qhrrof94IKpnuRg_4d

Your S6 will work with LTE data fine.  However, it doesn't have the required VoLTE features to work with AT&T's VoLTE service.  Up until this year, you could use LTE for data and still use the old voice protocols for voice/text.  That is going away.

If your S6 were an AT&T-branded S6 (model G920A), then AT&T has a software update that would upgrade it to being able to handle VoLTE, but that's not available for non-AT&T branded S6s.

The S6 is a pretty old phone.  You probably have gotten a lot of use out of it.  I had an S7 for over two years, and I replaced it with an S9 two years ago.  I do understand the large phone issue; I have to buy the smallest available phone for my wife.  She's using an S8 now, and wasn't happy that it was larger than her Droid Mini.

Thank you for the explanation!

For now, a temporary solution turned to be easy: I put the new SIM (came inside the burner phone) into my S6, and it works perfectly again.

I'm trying to understand, when they deactivate 3G (in Feb 2022), voice and text will no longer work, but 4G LTE will still work on it? Or will none of it work?

P.S.

I am not in the habit of replacing my phone every two years; this is only my second smart phone! But at least I gained a few months to find a suitable replacement. They Alcatel phone they sent is really bad.

Your s6 working again is probably short lived. Once ATT realizes you have reactivated a non supported phone, they will suspend your service. That could happen at anytime, you probably don’t have until February. Once the final shutdown is complete, nothing will work because that s6 won’t be allowed on the network.

LOL...I've got an AT&T S4 Mini that works GREAT (and is whitelisted for use on the new system)

And it uses Android 4.4....go figure!

@phone_afficianado  I think back to my galaxy S3, compared to the specs on the cheap $40 prepaid phones, and more current S class flagship phones, and laugh. 

If you just use it for call and text, it's plenty.  But I work off my phone, as many do.  It's a portable computer, not just a phone.