In reply to a post by EddyTheDog:

Never considered Vodafone in need of HD voice to be honest, i've always had great call quality with them.

EE and Vodafone standard voice is very similar - ie good, and no real difference between 2G and 3G. This is probably because in the 2G days in the mid 1990s they all rolled out the higher quality codec, which took more network capacity to deliver but made clearer calls. O2 was always the worst.

HD Voice on all networks is significantly better, and hopefully as 4G voice calling rolls out (VoLTE) then we will get HD calls between networks.