Wait, can you clarify? Are you saying that, AT&T, the most stubborn carrier with VoLTE will actually allow it on an unlocked phone? That's historical if true. For some reason, AT&T always has a bug up it's **** about VoLTE (and VVM for that matter). They REQUIRE a branded phone.
Meanwhile, the Nexus 6p/5x chug along on T-Mobile all day on VoLTE out of the box. Even wifi calling (limited to only the iPhone on AT&T). It's aggravating how militant AT&T is about this. I get it, the code needs to be there. It's there on the 6P, AT&T VoLTE isn't Why. (6.0 supports it out of the box).