Which three are supervisory signals? (Choose three)

A.
busy

B.
on hook

C.
off hook

D.
call waiting

E.
ring

Explanation:
Supervisory signals involves the detection of changes to the status of a circuit. In other words, supervisory signaling is used to indicate the state of a circuit. Once these changes are detected, the supervisory circuit generates a predetermined response. There are three different types of supervisory signals, which are:

1. On-hook

2. Off-hook

3. Ring

When a telephone handset is in the cradle, the circuit is said to be on-hook. In on-hook state, the circuit is said to be open, thus preventing the current from flowing through the telephone.

When the telephone handset is removed from the cradle, the circuit transitions to an off-hook state and there is a current flowing through the electrical loop. When the telephone network senses the off-hook state via the current flow, it provides a signal in the form of dial-tone that it is ready to accept the call. When making a call, the caller receives a ringback tone from the telephone switch, which alerts the caller that the telephone switch is sending ringing voltage to the called party. It is important to know that only the ringing that the recipient (the called party) hears is the supervisory signal; the ringback tone that the caller hears is simply a call-progress indicator and is not a supervisory signal.