A ringdown sends a signal to a phone operator or triggers an automatic circuit when a phone is activated. In a system for contacting operators, the ringdown is largely outdated, as automation has replaced many operator functions and it is no longer necessary to use an operator to place a call in many settings. Circuits that operate with a similar signaling method, where taking a phone off a hook completes a connection, are used in a variety of applications.
The original example of the ringdown was seen in connections where the user needed to signal an operator to use the phone. When someone lifted a phone off the hook, it sent a signal current to a phone company technician, who could place a call at the direction of the user. After the call was complete, people would have to “ring off,” telling the operator they were done and hanging up the phone to cut the connection. This technology is no longer in widespread use.
Similar circuits where a ringdown communicates directly with a circuit, not an operator, can be seen in a variety of settings. In such systems, when someone picks up a phone, the circuit activates and another phone rings. Multiple units can be connected to the circuit to allow a single caller to ring through to multiple phones at once for conferencing purposes. This creates, in effect, a dedicated line, with a direct connection between the two or more phones.
One example of a ringdown circuit can be seen with courtesy phones installed in businesses. People anywhere in the store can pick up one of these phones to be connected with a store clerk who can assist them. Such connections are also used by emergency services, courtesy taxis, and so forth. One advantage to such systems is that people don't need to know and dial a number to get connected, as just picking up the phone puts the call through.
In addition to being used as a courtesy and public safety measure, ringdown circuits can also be useful in situations where people frequently dial numbers and want a fast connection. A classic example can come up in securities trading, where floor traders may have phones at their desks that connect instantly to an office, information line, or other resource. Precious time can be wasted dialing and waiting for a call, while the ringdown allows for an immediate connection.