I don't even know what HD voice on a mobile is.
Same quality as FaceTime audio but using normal phone calls. Currently you have to be on the same network and both in a 3G coverage and not drop down to 2G whilst in the call.
Not all handsets support. iPhones from 5 onwards so.
All UK networks ( EE, 3, Vodafone and O2 ) now support but EE was first when orange, and O2 was very recently.
Hope is that with 4G voice calling the networks will inter operate.
plusnet unlimited fibre
80/20 product - Installed 2 June 14 - April Sync 57 / 11 with G.INP
16 years UK broadband (Since 1999 ntl:cable trial), Asus RT-AC68U & HG612 - BQM - Speedtest