it depends what is meant with "HD Voice" but if "HD Voice" means WB-AMR then it is available for GSM and WCDMA networks. Even calling from GSM to WCDMA in wideband quality is possible. It is of course another question if the installerd HW in the network does support WB-AMR in both systems. Or if the operator enables it in both systems. Or if there are mobile terminals supporting GSM and WCDMA.