johnr: Just cause a handset supports HD voice does not mean it's enabled as mentioned above by Sbiddle
And it's not something that can be enabled by the user? Is the idea of including it the possibility for it to be enabled via future firmware updates?
EDIT - if it is something either enabled or not by the manufacturer, how can the same phone be supported by one network but not others? Eg, Vodafone clearly lists my phone (Xperia SP) as "HD Voice compatible".