Well, just got off a 1/2 hr call to them.Just had a chat with AT&T. You do need to contact them and give them your IMEI to get VoLTE working. Mine now works.
Indeed I now have VoLTE service on my 2XL!
The techs had no idea how to do it. He kept trying to enable wi-fi calling (didnt have the resources to do that, neither did the android specialist).
So I asked them about HD Voice (ATT version of VoLTE) again said he didnt have the resources, so I suggested that my account/phone needed to be provisioned for it, he took my IMEI, changed it in the system, and after rebooting the phone, I now have LTE (instead of H) when calling.
Now, the tech seemed quite amazed that it actually worked, which I found funny, he was truly excited it happened, I could hear it in his voice.
He apologized a few dozen times throughout the call because it was taking so long and that they didnt have any documentation.
In the end, its working now, so I am happy.
Oh and calls sound a LOT better now for the folks that have HD Calling.