VoIP systems negotiate codec choice on call initiation, generally using a "best common denominator" rule. Even when only companded PCM is available (PCM-a, PCM-

VoIP systems negotiate codec choice on call initiation, generally using a "best common denominator" rule. Even when only companded PCM is available (PCM-a, PCM-µ), 64kbps will be used since it's the universal norm on the TDM (conventional) telephone newtwork. Unfortunately, as a capacity measure GSM specifies very low bitrates for voice connections, as low as 5kbps in the worst case and 10-20kbps typical. This requires the use of high-efficiency vocoders like CELP variants that are sufficient for intelligible speech but, well, only for that purpose. "HD voice" is exactly VoIP and with few exceptions is only transported over SIP or a SIP-like protocol using the existing RTP negotiation mechanism, which usually ends up selecting 64kbps companded PCM (same as a landline phone). Increasing use of VoLTE, which is essentially an optimized form of SIP designed to "combine" session control features with LTE for lower overhead, has made this pretty common as HD voice support is standard from VoLTE vendors. There was such a thing as HD Voice over 3G using a similar mechanism that leveraged HSPDA but it was never very common, at least in the US. VoLTE will quickly become the only way to make cellular phone calls in the US which we can expect to make HD Voice pretty universal. Right now it can be spotty when calling between networks, depending on how their peering is set up.