What is the Operational Definition of Telephony Quality of Service?

Quality of Service (QoS) is primarily applied to internet telephony (IP telephony, VoIP).

In VoIP (Voice over Internet Protocol) telephony the cable frequently shared with internet connection.

If internet connection uses/occupies too much of cable's transmission capability by downloading or uploading files, there will be not enough cables resources to transmit telephony signal effectively thus making conversation impossible or with low hearing rate.

QoS reserves certain amount of cable's capability only for telephony, thus no matter how much internet connection is used, there is always enough room for plenty of simultaneous telephone conversations.