No mobile data after giving IMEI to att

If you are receiving messages from AT&T that your phone will not work once 3-g shuts down, the message is correct. AT&T 4g is their 3-g/HSPA+  Network.

It is shutting down.

You must have a phone that is capable of their 'HD Voice' or voice over LTE service.

Here is the test. Make a phone call. While you're on the phone call look at your notification bar if it shows 'Volte'  or an H or H+.   The H or H+ would indicate your phone has dropped to a 3-g network for phone calls. Your phone is not compatible with AT&T network and must be replaced.

You can either take the cheap free phone that AT&T is offering, which comes without any strings catches or obligation, or purchase your own phone that is on the AT&T whitelist.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/ecms/dam/att/consumer/help/pdf/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf&ved=2ahUKEwiyzPG67tTwAhXaGFkFHSp9CHwQFjAAegQIEhAC&usg=AOvVaw3sCP2kJRUevj0ARWKoc1nJ

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/ecms/dam/att/consumer/help/pdf/Devices-Working-on-ATT-Network.pdf&ved=2ahUKEwi09tD0kfXvAhVZFFkFHdwQAiIQFjABegQICBAC&usg=AOvVaw2Fi4GrglhetKH_v9vTKe4N

My phone has worked fine on the network for a year and a half. The day I gave them the IMEI, my mobile data stopped. It has nothing to do with 3g or 4g. I only called because I don't need their free phone. They didn't even know what phone I currently have until I told them. Att screwed something up and I can't find anyone there to fix it. 

You don’t get it. ATT will no longer activate an unsupported phone. An unsupported phone already in use will keep working but they won’t activate one. If you are using an unsupported phone, by providing the correct imei, you essentially are trying to activate a new phone. You haven’t identified the actual phone so no idea if it’s on the supported list or not but if it isn’t, that’s likely the reason it quit working.

I just solved it.

Welcome to the Mint fam. From here on out, things are gonna rule for you.
Stuff like pricey phone bills, overpaying for wireless and inferior corporate mascots will melt away like gas station nacho cheese.

Hope that works out for you but hopefully you realize Mint is a virtual network provider. They don’t own or operate their own network, they use T-Mobile.

@Mjibba

It has everything to do with your IMEI being a phone not permitted on the network. 

ATT 3 major networks have been very public about shutting down its 3g network.  

Phones must work on LTE to work.

What part of "my phone works on 4g" is so hard to understand...

4g is not LTE.

AT&T 4G network is 3-g, and it is shutting down. Your phone must be capable of working with voice over LTE and it is not.   If you had bothered to look at the two lengthy list of compatible phones oh, you would see whether or not your phone was on the list. Phones including the iPhone 5S will no longer work on AT&T. A lot of AT&T older Android phones will not work on AT&T anymore. Just because your phone is capable of LTE for data does not mean it is capable of LTE for Voice and text.

Your phone has been flagged by its IMEI which is why you are receiving messages that your phone needs to be replaced.

Your phone has been flagged by its IMEI which is why you are receiving messages that your phone needs to be replaced.

Does ATT have a time machine I don't know about? Somehow they flagged my IMEI four weeks ago, but I just gave it to them one week ago? 

Quit being a corporate bootlicker. Att has screwed this up and lost myself and my family  as customers because of it. Somehow customer service, escalation agents and the technical support people I have spoken to over the past week can't give me an answer or bother to call me back when they say they will speaks volumes about this company. I'm done. Enjoy your day, shill.

I just ran across this thread and had some observations about my phone.  AT&T has condemned it as not compatible.  However, when I put the SIM into the cheapo phone they sent me, then back into my newer, disapproved one, it says 4G-HD and works real nicely with voice and data at the same time.  For a while.  Then after a while, it appears that AT&T downgrades it to 4G only and when I make a call it goes to 3G.  So my phone apparently IS capable of working with the newer protocol if AT&T allows it.  I suspect that AT&T is doing this to force us to buy new phones from them, even though we already have phones that can function well with the upgraded network.