Comfort noise is the quiet, synthetic background hiss a receiver generates to fill the silence when discontinuous transmission has stopped sending real packets. Without it, the gaps during DTX would be dead digital silence, and listeners reliably interpret total silence as a dropped call, so they start saying 'hello? are you there?'. The encoder sends a tiny silence descriptor (a SID frame, standardized as RFC 3389 in RTP) describing the spectral shape and level of the real background noise, and the decoder synthesizes a matching low-level noise so the transition in and out of silence is seamless. It is a small touch that makes bandwidth-saving DTX feel natural rather than broken.
