A very low-volume tone injected into the receiving end of voice over IP (VoIP) communications to prevent the listener from thinking the line went dead. There was always a slight background noise in analog phone lines. However, digital methods have replaced analog, and without the comfort noise, the line can seem disconnected during pauses in conversation, or when someone is on hold (without music) or when the other party puts the phone down for a moment. See
TDM-to-packet conversion.