LM-G820UM - Unable to make calls since last month.

Hello amirdhagopal, we're here to answer your questions about 3G sunset.

Your current unlocked device will not work on our new network. You'll find a full list of compatible devices. This is because they have different hardware that is not compatible with AT&T’s network (HD Voice, VoLTE compatible). If you purchased an unlocked device from another retailer, please note, they are not optimized to work on AT&T’s network. This is important to remember when purchasing a non-branded, unlocked device and why you see different model numbers.  

Just to make you aware every carrier is upgrading their network this year. In order to know if your number can be port out you can call customer service at 800-331-0500. You can also upgrade your device with AT&T. 

Please reach back out if you have any further questions or concerns.

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Carlton, AT&T Community Specialist 

Since the LM-G820UM had been working on LTE and I have VoLTE enabled on the phone, I would love to have a specific reason my phone isn't compatible.  For instance, it a frequency issue?  What is required and what it the LM-G820UM limited to?  Thanks!

@ctmax45

Your phone is not HD voice compatible with AT&T VoLTE.  It's software and imei exclude the sprint branded phone.

LG did not certify newer phones or provide software that would allow it to work on other networks because it is no longer in the mobile business. 

Phones must be un-branded, or AT&T branded. 

Non-carrier branded phones that will work on AT&T now that 3-g is gone

You might have VoLTE "enabled" in the phone's settings, but that doesn't mean that AT&T recognizes it and uses it, or ever used it.  It's probably still trying to drop to 3G for phone calls, but with no 3G frequencies to find anymore, then it just won't work.  VoLTE is not the same as the data running on LTE.

That's just the way it is.  That phone has several variants, and most of them are not on AT&T's compatibility list after the 3G shutdown, and AT&T is not going to do anything to get you on it.

Is there more to the model number after the "UM"?  There is an "LM-G280UM1" for AT&T, a "UMB" for Verizon, a "UM0" for Sprint, and a "UM2" for Canada.  Oh, and then there is the "LM-G280QM7" which is the US unlocked model and is on AT&T's compatibility list (with a software update).

As to the reason yours wouldn't be compatible, LG never paid and submitted that version of the phone to AT&T for certification.  And at this point, LG never will either since they left the smartphone business just over a year ago.  LG's only responsibility now is to whatever warranty and support periods are left - and I'm sure they are just waiting on those to run out.

And I see @formerlyknownas beat me to most of those points.  I had my post sitting open for over an hour while doing other things before I actually posted it.

It is an unlocked Sprint phone.  At least that's the splash screen that comes up on boot.  It doesn't show any other letters or numbers though.

@ctmax45   as you wrote, it's a sprint phone.   It can't work on at&t as it won't make calls over LTE.

Either replace the phone, or switch to a different service provider.