One of the greatest concerns in the move towards voice over IP (VoIP) networks has been the issue of quality. Call-quality has typically been synonymous with voice quality. However, there are many other factors in addition to "voice" that affect someone’s perception of the quality of a call.
While hearing intelligible words is the basic driver of a useful phone call, the experience of call-quality includes much more than just the voice heard. The sounds that are heard when there is no voices on the line are often overlooked yet are important contributors to the perceived call quality. With analogue based connections, there is a constant stream of information going each way. Background noise, speech, whatever. However, with packetised audio connections, if there is no voice on the line, nothing is sent.
The trouble is that absolute silence on a VoIP line when someone stopped talking is so stark, so dead, so absolute, that people at the other end find it unnerving and fairly freaky.
The solution is to turn on comfort noise; technology that sits in a telephone receiver and blends selected sound frequencies at just the right volume to create the impression that the phone line is still alive and to satisfy the user’s need to know that someone is listening. These non-voice sounds are typically called background noise. Two technologies used by packet voice systems — echo cancellation and silence suppression —have been employed to artificially replace the background of a call with what is commonly called comfort noise.
Note: Although I have mentioned that the issue of silence affects VoIP, it is a consideration with any form of packetised voice. For example, GSM, CDMA, W-CDMA etc.