G.711 is the 1972 ITU telephone codec and the lowest common denominator of digital voice. It is companded PCM — 8 kHz sampling, 8 bits per sample, 64 kbps — using either the mu-law (North America, Japan) or A-law (rest of world) curve to give better low-level resolution than linear 8-bit would. The audio is narrowband and unmistakably 'telephone', but it has almost no delay, trivial CPU cost, and is understood by literally every VoIP and PSTN endpoint, so it remains the universal fallback in SIP and WebRTC when nothing better is negotiated. When you hear a call drop to thin, plain telephone quality, you're often hearing G.711.
