The mandatory codec is G.726 running at 32 kbit/s.
Bear in mind that unless the end to-end-path is using a high quality codec you will get little if any benefit from having it in use in only a part of the path. The world's traditional telco networks are still almost exclusively G.711 - 8 kbit/s sampling and 8 bit PCM with non-linear quantisation. And if the distant end is a cellular phone the codecs they use usually give even lower voice quality.
Some telcos have offered HD voice For example BT in the UK had its 'Broadband Talk' VoIP service which could provide 'HD' capability with its custom DECT phones parented on its early Residential Gateways (BTHomeHubs 1, 1.5 and 2) when communicating with a compatible other end. It didn't make much headway in the market place. In fact the VoIP and DECT capabilities were not present in any of its later HomeHubs (3, 4, 5 and 6).