Quality of service (QoS) is the name given to the subjective performance of a VoIP system, in terms of audio quality, reliability of calls, uptime of the phone system, etc. Since most phone systems are highly reliable, this term has come to be applied mainly to the voice quality of the phone calls, as perceived by the users.
A high-quality phone call will have high QoS, and be free from problems such as distorted audio, clicks and other noise, dropouts (unwanted silence), disconnects, or one-way audio.
QoS is very important for business phone systems as poor phone calls can seriously affect the functioning of the entire business, particularly those that are reliant on frequent customer interaction. Small businesses often cannot function if the phone system is not working properly.