Call waiting

Call waiting is a telecommunication service offered by a telephony service provider to a subscriber by which the subscriber may suspend a telephone call already in progress to accept a second call. The subscriber may switch between calls, typically by using the hook flash signal.

Call waiting alleviates the need to have more than one telephone line for voice communications.

In North America, the NANP uses the *70 star code prefix to suspend call waiting for a dialed call. A stutter dial tone confirms the de-activation.

On most European telephone networks, and GSM or UMTS mobile phones, call waiting is activated by dialing the following codes:

A voice announcement, tone or a message on your phone's screen will confirm the service status.

Call waiting in Europe uses an "R" (recall) button on the phone. This performs a similar function to a North American hook flash button but is much shorter duration, typically 80ms to 100ms, vs. 250ms in North America. In some networks, pressing R toggles between the calls, similar to North America. However, in most countries there are further options:

Type II Caller ID also works with call waiting.

Since the waiting call creates an audible signal (for example, a 440 Hz beep every ten seconds in North America), call waiting can cause dial-up Internet access connections to terminate, unless the modem supports the most recent V.92 modem standard. For this reason, call waiting is often disabled on shared voice lines used for dial-up modem or fax purposes. However, Call Waiting has no impact on DSL connections.

Call waiting was introduced to North America in the early 1970s when the first generation of electronic switch machines built by Western Electric, Electronic Signaling System 1 started to replace older mechanical equipment in the old Bell System local telephone companies. At first, some smaller municipalities had it only on a specific phone exchange (e.g., phone customers in Trenton, Michigan initially had to have a phone number starting with 671 to have call waiting, since 671 was at that time the only exchange in that area served by one of the new ESS switches), but as demand for it became more widespread, it eventually became available on all phone exchanges as the older equipment was phased out.

...
Wikipedia