Glossary Call quality
What is Codec?
Also called g.711, g711, opus, g.729, audio codec
A codec, from coder-decoder, is the algorithm that turns audio into data small enough to send and then turns it back into audio. The choice of codec sets the trade-off between how much bandwidth a call consumes and how good it sounds, and both endpoints must agree on one before a call can proceed.
The ones you will actually encounter
G.711 is uncompressed telephone-quality audio at roughly 64 kbps per direction; it sounds like a traditional phone call and is the safe default. G.729 compresses to about 8 kbps at a cost in quality, and mattered when bandwidth was scarce. Opus is modern, adapts its bitrate to conditions and handles wideband audio well, which is why it sounds noticeably better than a normal phone call when both ends support it.
Why HD voice sometimes does not happen
Wideband audio only works if every hop supports it. Two handsets on the same system will often negotiate Opus and sound excellent, while the same handset calling a mobile drops to narrowband because the call crosses the public network. This is normal and not a fault: the lowest common denominator wins.
Transcoding is the thing to avoid
If the two ends of a call cannot agree on a codec, something in the middle has to decode and re-encode the audio. That costs processing, adds latency, and loses quality that cannot be recovered, because each compression stage discards information permanently. It is the audio equivalent of saving a JPEG repeatedly. Where you can, arrange for the same codec end to end, and be aware that some cost-saving routes work by transcoding to a low-bitrate codec you did not choose.