HD Voice (AMR-WB) is supported on VoLTE as well as on 3G networks by the Nokia 8. As more operators are introducing EVS (Enhanced Voice Services) on VoLTE across Europe, can Nokia support this codec as well on the Nokia 8?
Vodafone is using it (https://www.vodafone.de/hilfe/mobiles-telefonieren/crystal-clear.html) in Germany as well as in other Vodafone countries (e.g. NL). Also T-Mobile is offering this to customers in various countries.