Thanks for your replies. My V20 is a BYOP on AT&T Tracfone. Even though my HD voice button isn't disabled and can turn it on, it does nothing.
My SIL who is on Straight Talk, Vzw (Tracfone) has HD voice on a LG Rebel 2 and it works. When it's on the letters "HD" is displayed in her contacts list.
I have talked to her without it and with it on and it sounds great with it on. She didn't hear my HD since I don't have it. So I do think cross network interoperability is working. At least in my neck of the woods.