Which signal would indicate the end of a call?

• A. A closing remark from the caller ends the call.
• B. The line goes silent and the operator disconnects automatically.
• C. A designated end-of-call signal is transmitted to both parties.
• D. The caller hangs up or returns the handset to the cradle.

Answer: (D)

In sound-powered telephones, there’s no electronic “end of call” signal. The call ends when the circuit is physically opened by the user: the caller returns the handset to the cradle (hangs up). That action breaks the circuit, signaling to the other party that the conversation is finished.

The other ideas rely on coordination signals or automatic disconnections that aren’t part of the basic sound-powered system. A closing remark might come at any time and doesn’t guarantee the line is cut. An automatic disconnect by an operator isn’t how these systems typically operate, and a special end-of-call signal would require additional signaling hardware not inherent to standard handsets. The only dependable end is the handsets being placed back in the cradle.