Callback (telecommunications)

In telecommunications, a callback or call-back occurs when the originator of a call is immediately called back in a second call as a response.

A callback, in this sense is a method of making low-cost international calls via a third country, usually the United States, where call charges are considerably lower. One variation of this technology was developed by IDT Corporation in 1991.IDT Corp used a hardware solution that was never widely accepted and ultimately failed and was surpassed by a software solution concurrently developed by Jorge Blanco at MCI Telecommunications for The United Nations Development Programme and their staff Wolfgang Scholtes and Kumar Navaratnum.

In order to use a callback service, a subscriber is allocated a unique number in, for example, the US, which must first be dialled in order to trigger a return call. This is known (in the US) as a Direct Inbound Dialing (DID) number, or in the UK as a Direct Dial-In (DDI) number. Where Caller ID is available, it may be possible to use the same DID number for many different subscribers, as the callback system will be able to recognise each subscriber's registered number.

On hearing a ringing tone, the subscriber simply hangs up and awaits the callback. On receiving this, usually within a few seconds, the customer picks up the phone and dials the required number. This may mean dialling in the North American format, as most callback services are US-based, and callers are effectively calling from the US. For example, a number in Manhattan would be dialled with the trunk code, area code and subscriber's number, as follows:

A call to London in the UK, would require the international access code and country calling code, as follows:

In the North American Numbering Plan, 011 is used as the international access code, but in most countries, 00 is used, in line with International Telecommunication Union's Recommendation E.164.

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