GSA reports that ".. 116 mobile operators in 75 countries have commercially launched HD voice services .. more than 30% higher than a year ago. Mobile HD voice uses Adaptive Multi Rate Wideband (W-AMR) technology, enabling high-quality voice calls in supporting mobile networks and an improved user experience for calls between mobile phones supporting the feature"
"The majority of operators offer HD voice service over a single radio access network, however some offer the service over more than one network system: The system technology totals are:
- 105 HD voice operators use 3G/HSPA networks
- 10 HD voice operator use 2G/GSM networks
- 11 HD voice operator use 4G/LTE networks"
See "116 mobile operators launched Mobile HD voice in 75 countries; 11 operators launched with VoLTE" - here.