Class of service tone

The class of service tone is an audible tone played to a telephone operator indicating a special class of service for the calling party. It is commonly used to indicate the caller is using a pay station.

While class of service tones have since been superseded by electronic indications, such as those used on TSPS consoles, the class of service tone provided crucial information to operators in the days when cordboards remained common for dial "0" operator services. Class of service is needed to distinguish coin phones from other lines so that calls cannot be placed through the operator without a coin deposit. The class of service typically is a short burst of low tone or high tone and, when used, is audible to both the calling party and the operator.

The actual tone is generated by the dial "0" trunk circuit which puts the tone on the sleeve lead when the operator answers. Because the sleeve is a control lead not generally used for voice transmission, such noises are not usually audible. However, traditional coin phones have a capacitor between the sleeve and tip, and the capacitor makes the tone audible on the line. This proved to be an efficient and economical means of providing class of service indications to the operator.