It should -- especially if the phone is connecting to Band 12. It was only disabled in phones that had issues making a call while on Band 12 since this could prevent someone from calling 911.

You seem to have that, or at least described it, backward. Because low frequencies travel farther than higher frequencies (band 12 is a low frequency, bands 2 and 4 are high frequency), in some areas on T-Mobile you might only be able to get Band 12. Since Band 12 is LTE only, 2G and 3G are on Bands 2 and 4, you might be in a situation where you can only make a call if VoLTE works -- you can't drop down and make a call on 2G or 3G. Band 12 is opening up new areas for T-Mobile because of it's range, where they didn't previously have service -- and where it would be very expensive (and likely unneeded) for T-Mobile to build new towers. By contrast, if you have an LTE connection on Band 4 and can't make a phone call, you should be able to drop down to 2G or 3G to make it if the phone doesn't support VoLTE.Because of this, the FCC has had some manufacturers disable (Motorola) disable Band 12 on phones that weren't VoLTE certified with T-Mobile; as, since you can't drop down, you'll show you have a connection, and can even get data, but can't make a phone call. More particularly (and the reason the FCC cares), you won't be able to call 911. Since the phone "has a connection", it won't "roam" trying to make a 911 call as it normally would, if you had no connection.So, to repeat, VoLTE can work on any band over LTE; but it is critical on Band 12 as you might not be able to drop down to 2G or 3G to make a call -- meaning at those times you can only make a call using VoLTE.