Not compatible

Did you give ATT the imei number of that phone?  Because that phone is compatible with VoLTE.  See the list

https://www.att.com/ecms/dam/att/consumer/help/pdf/Devices-Working-on-ATT-Network.pdf

I have been using the phone on AT&T plan for almost a year now...this is a new thing as of 15 mins ago...

As I said did you have your IMEI changed when you got the phone?  The phone will work fine for you after the 3g shutdown.  That is why its on the official list I gave you.

When I activated it last year they did take the IMEI number..maybe I have to go in and give it to them again.

Dont worry about it...  The phone will work just fine after the switch which is still 18 months away

Thanks for the information.

@glimmerman76 You posted a link to a document about phones that DO work on ATT - my G6 isn't there and I've never noticed it not working (...).

On a different message thread about phones "soon" NOT working on ATT you posted a link to another document about phones that will work on ATT after 3G goes away; my G6 is also not in that document. As one on the outside looking in, I could easily begin to suspect that the lists are incomplete/incorrect.

Commentary aside: Since ATT is about to have a widely watched earnings report and conference call and given the number of messages beginning to appear on these forums about this incompatibility of (hundreds of?) thousands of customers' phones with ATT, I would suspect that some customer-oriented entity internal to ATT has not been talking enough to some ATT-business-oriented entity internal to ATT...

However, your posts are still appreciated completely since they contain real information that ATT itself should have overtly given its phone customers.

That list is COMPLETE.  Those are the phone that have been certified as of the date on the PDF.  

Your phone does not support VoLTE because MOTO aka Lenovo was not playing nice with the carriers the year the phone was released.  You will notice that they are playing nice with the g7 series and the Z4.

Hi does my LG k51 will work ever?

We'll be happy to clarify the compatibility for you @kimjaeyoung3.

The LM-K500QM model of the LG K51 should work on our network. Here's a full list of the devices that should work on our network.

Are you trying to initiate service on our network? Please start a new thread with more details about what you're trying to do so that we can further assist you; since this is an old thread, it may be closed.

Thank you for reaching out to our AT&T Community.

Darais, AT&T Community Specialist

Its a LG LM-k500mm

Thank you for responding @kimjaeyoung3.

That model isn't approved to work on our network; you can purchase one of our approved phones from our online shop. 

Please let us know if you have any additional questions or concerns.

Darais, AT&T Commnity Specialist 

No one here can approve a phone if it’s not on the list it won’t work and no one can change it 

the data works just completely fine the Voice don't

And that's why it's not compliant.  No voice as it does not support volte.