How does voLTE and Band 12 work for T-Mobile and can Cyanogen/AOSP allow us any control over this? How does voLTE Certification work? My understanding is that only phones on this approved list by T-mobile are supported on Band 12. Band 12 is the new Extended Range LTE band and T-Mobile is investing heavily in. My understanding is that voLTE Certification is something that T-Mobile does to ensure that a phone connected to Band 12 but out of range of other T-Mobile services to fall-back on (ex. 2G, GSM, and 3G HSPA+) can still dial 911.. If the phone lacked this ability, when they went to dial out on Band 12 they'd find they couldn't. Even if I'm wrong, VoLTE seems to have something to do with data and voice integration.


  VoLTE calls already go through faster (twice as fast as other calls), and while on VoLTE, customers can enjoy wicked-fast LTE data while they’re on a call. Customers on the old carriers’ CDMA networks still can’t do that. Today, 27 million T-Mobile customers on 40 models of VoLTE-enabled phones are making over 300 million calls a day— well over half of all calls on T-Mobile’s network.


The voLTE process is expensive for the OEM and almost sounds like a shake-down by T-mobile:


  Speaking off the record to one major smartphone vendor, we have learned that there are very real costs and complexities associated with obtaining that certification, and it must be done on a device-by-device basis.


And, one of the OnePlus developers said this


  In terms of technical specs, supporting VoLTE is actually not so hard, but there's still a long way to go for users can actually use it. Why? Because VoLTE is carrier-specific, we needs to meet the carrier's requirements firstly then our users will be able to actually use it.


As if to say, we're only waiting on testing and certification.. Further, because T-Mobile had to ask Motorola to remove Band 12, I take it that the OEM (client side) has to disable B12 and not the carrier (server side).

Does that mean Cyanogen or other AOSP's can re-enable B12? I would think so, but even small OEM's selling unlocked universal phones like OnePlus have commented that they're having problems with voLTE Certification, so I guess it's not entirely software?

I understand T-Mobile's concern, but I simply don't care. Give me LTE if Band 12 is available and if my phone can't connect, well at least I've got decent data and I can use Google Hangout Dialer.


What happens with a phone that has Band 12 but not voLTE?
Can a rooted phone running AOSP influence this, or give me control over it?
So I'm going to answer as someone who has a non-VoLTE phone in a market that has T-Mobile LTE, but no 2G or 3G coverage. 

With an alternate ROM (OR a way of editing the modem settings or keeping an older ROM installed) on your Android device, enabling Band12 will be no issue. It's possible with a bunch of Android devices already to enable bands that are available, but disabled by the OS.

What you'll have happen is exactly as described. You phone will get data service, but you will be unable to send or receive calls. You will get SMS and MMS messages just fine. My phone (Nokia Lumia 830 RM985) doesn't support all of T-Mobile's LTE bands, and HSPA bands. This means that in the market I'm in right now, I can't make or get calls as long as my phone is connected to a t-mobile tower. I can see when a call is trying to come in because my service will drop and my phone will roam, but the process takes longer than t-mobile's system allows and it times out first time.

Same with placing a call. First attempt times out and is registered as a dropped call. Second attempt drops to roaming and the call is placed.