My and my wife's phone WILL NOT do HD Voice. We receive calls from other people i.e. Verizon, T Mobile and even Xfinity Landlines and they pop up as HD voice for them. But, not us. My wife had an S8 but just got a S9 for her replacement device from insurance. I also just got a new SIM card as my IMS registration somehow got unregistered. Just tonight I drove over by a corporate AT&T store as they usually have a tower very close by. Speed tests are coming out at over 100Mbps and close to 20Mbps upload but you guessed it, no HD voice with either me or my wife. All AT&T phone support says is VoLTE is provisioned and so is HD Voice.
Here is the other thing. Ever since the One UI 2.5 update for our phones the "Advanced Calling" option has disappeared, and so have the "LTE Settings" where it was located.
I have been made more aware of HD Voice because I started using the Google Dialer and it tells you when the other person is coming through with HD Voice. The Samsung dialer does not tell you.
I am not sure whether this is AT&Ts fault or Samsungs but I am not a happy customer of either right now and I have never had a problem with either since I have had either for many, many years.
So, HD Voice on my phone is working fine but AT&T is doing something with AT&T to AT&T calls which is really (explitive) me off. So, this thread is moot now.