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Automatic number identification (ANI) is a feature that allows a telephone to show the number of the calling party. How this information is provided is determined by the service provider. This allows users to screen their calls. ANI is also used by emergency call center dispatchers.
ANI was created by AT&T for the purposes of internal long-distance billing. It is separate from caller ID, which is a newer service that serves a similar function, but uses different underlying technologies. In the United States, ANI is part of Wide Area Telephone Service (WATS).
ANI describes the two-way radio selective calling functions that identify transmitting users. Customers purchase WATS as it employs a provision for prospective customers to call the number toll free. Customers are issued a separate toll-free telephone enclosing numbers beginning with special area codes. The subscribers to these numbers are termed as Inward WATS subscribers.
Unlike caller ID, ANI is resistant to blocking and is offered to commercial customers who are interested in knowing the numbers for the calls they receive.
The ANI service is provided by sending a digital multifrequency tone along with a call. The data burst has a unique number ranging from one to eight digits in length. A caller's telephone number is captured by an ANI service even if caller ID blocking is activated in it. The telephone company switching office at the destination provides the caller's telephone number to ANI delivery service subscribers. Residential subscribers can also access ANI data through some third-party companies that charge for the service. Calls through VoIP service and calling cards send a working number as ANI.