Band 12 phones must support volte or T-Mobile asks the OEM to disable it. Has happened to some Motorola phones (Moto e?) that shipped with band 12 then revoked it, believe the one touch idol 3 too. Has to do with 911 emergency calls and potential big fines for T-Mobile. Band 12 is LTE data only unless volte is supported by the device
You can still use a band 12 device without volte, it will likely be an imported device. Calls will not work if you are actively on band 12 though and there is no other available signals. T-Mobile does not have low frequency hspa so there are scenarios where you will have data and no voice.
Tried T-Mobile for a bit in my area last month and we have some band 12 around, but lack it in some critical areas for me (which is strange, I figured if it was here it would be everywhere here). Would get dropped calls with the phone trying to cling to band 4 LTE / aws hspa
I really like what T-Mobile is doing though, and I hope they can get more than 5mhz of band12 or more low frequency band whatever. It gets congested fast. We have ridiculously fast 20mhz band 4 LTE here and it is wonderful when you are in range. Back on att gophone for now. Learned a lot of stuff about cell frequencies while trying out T-Mobile so there's that !
I have ringplus on my tablet now, but if my needs grow I say T-Mobile is a no brainier for less critical things like tablets, especially with binge on