On AT&T you also need to be using the nxtgenphone APN.
APN Descriptions and Characteristics | AT&T Developer
Requirements for VoLTE calls:
- Both sending and receiving phone must physically support VoLTE in their hardware and firmware
- Both sending and receiving phone must be configured for VoLTE via APN
- Both sending and receiving phone must be connected to a VoLTE enabled tower
Any one of these missing from either side of the call = no VoLTE.
VoLTE is a protocol. AT&T should not be selectively blocking enabled phones whether carrier locked or not. It is a standard. Lumia 950 and 950 XL of any variant are enabled in the hardware and the OS firmware (independent of carrier firmware) for VoLTE.
This is partially correct. For VoLTE calls to happen only the sending phone needs to be VoLTE capable. If the receiving phone is not VolTE capable then the call can still happen over LTE. I own ATT enabled VoLTE phones and ATT feature (nonsmartphone) phones. When i call one of my feature phones from my VoLTE phone, the call carries over LTE, i.e. on the VoLTE phone i never see the LTE symbol change to 4G. Yes, nxtgenphone APN is a must, but both phones in a call are not required to for it to be carried over VoLTE, only one. This is because both towers (sending and receiving) are doing the job of this "voip conversion". The reverse is possible too. When Non VoLTE phone calls VoLTE phone, the receiving phone carries the call over LTE. Again, the towers are doing the magic.
Now, the HD Voice is a different thing. ATT tier 1 support folks need to decouple (meaning to quit making them the same thing; there is a difference between VoLTE and HD Voice) VoLTE from HD Voice. Yes, HD Voice is one feature from the VoLTE protocol. Only for HD Voice two phones are required for a "HD Voice call" to happen, but not for VoLTE. The high bandwidth audio codec for HD Voice to happen is embedded in phone's LTE modem, hence the need of two phones to work.
Analogus to VoLTE is Skype, because Skype uses VOIP (voice-over-IP) protocol. VoLTE is one form of VOIP.