Codecs & Voice Quality
Voice Codecs Compared
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
A voice codec is the method used to turn speech into data and back again. With the older codecs, using less bandwidth means worse sound. Opus largely escapes that trade, and on modern connections saving bandwidth is almost never worth losing quality.
How it works
Your phone samples your voice thousands of times a second and has to send those samples as data. A codec decides how much detail to keep. G.711 keeps everything, using about 87 kbps per call once network overhead is counted. G.729 compresses hard, using roughly a third of that. Opus adjusts itself to the connection and can carry a wider range of frequencies than either.
The two ends negotiate which to use when the call is set up, choosing from what both support. You rarely pick manually — but you can usually see and restrict the list, which matters when something specific needs the uncompressed option.
Why it matters
The historical reason to compress was bandwidth, and that reason has largely disappeared. A G.711 call needs about 87 kbps, so ten simultaneous calls fit inside a single megabit. On any modern business connection, compressing to save that is trading audible quality for capacity you were not short of.
Where the choice still matters is machines. Fax machines and card terminals send precisely timed tones, and a codec designed to approximate speech treats those tones as noise it can safely discard. That is why a fax works over G.711 and fails intermittently over G.729, and why the fix for a misbehaving fax line is nearly always to force the uncompressed codec.
G.711 uses about 87.2 kbps and G.729 about 31.2 kbps. Opus adapts to the connection; at a typical 24 kbps setting it needs about 47.2 kbps. All three figures include the Ethernet, IP, UDP and RTP headers that per-call estimates usually leave out.
Related terms
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Work out your own bandwidth →Frequently asked questions
Which codec should I use?
G.711 unless you have a specific reason not to. It sounds best, it is universally supported, and the bandwidth it saves you to compress is bandwidth almost no modern business is short of. Choose G.729 only where capacity is genuinely constrained, such as a site on a slow or metered link.
Is G.729 noticeably worse?
Yes, though it is perfectly usable. It ranks lower on the standard quality scale and the difference shows on voices that are already hard to hear: background noise, poor handsets, unfamiliar accents. On a clean line with a good headset most people would not comment.
Why does my fax fail on VoIP?
Almost always because the call is using a compressed codec. Fax tones need to arrive exactly as sent, and compression designed for speech discards precisely what fax depends on. Force G.711 and, if the provider supports it, use T.38, a standard that carries the fax as data rather than as audio.
What is HD voice?
A wider range of frequencies than traditional telephone audio, delivered by codecs such as G.722 and Opus, and it genuinely sounds clearer once you hear it. Both ends have to support it, so a call to an ordinary landline drops back to the old quality regardless of what your phone can do.