We have HD Voice enabled on our Telnyx numbers, and we cant make outbound calls to mobile carriers without degraded audio. This is the response we got from Telnyx: "The incoming codec was G722 and outgoing to the receiving carrier for 1284 was AMR-WB.
Limitations are on all sides:
Livekit only allows G722 as a wideband codec
Receiving carrier only allows AMR-WB for HD Voice calls
Telnyx had to transcode but G722 -> AMR-WB are highly prone to artifacts" Here are some of the audio samples with bad audio when we enable HD Voice.