Automatic number identification
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Automatic number identification (ANI) is a feature of telephony intelligent network services that permits subscribers to display or capture the billing telephone number of a calling party. In the United States it is part of Inward Wide Area Telephone Service (WATS). ANI service was created by AT&T Corporation for internal long distance billing purposes, and is not related to newer caller ID services. Inward WATS is purchased by customers so that other telephone users (for example, prospective customers) can call the number toll free. The customer is issued a distinctive toll-free telephone number beginning with a special area code such as 800, or, more recently, 888, 877, 866, or 855. Subscribers to these numbers are typically called Inward WATS subscribers.
There are important differences between ANI and the separate, somewhat similar, but unrelated caller ID service. A caller's telephone number and line type are captured by ANI service even if caller ID blocking is activated. The destination telephone company switching office can relay the originating telephone number to ANI delivery services subscribers. Toll-free Inward WATS number subscribers and large companies normally have access to ANI information, either instantly via installed equipment, or from a monthly billing statement. Residential subscribers can obtain access to ANI information through third party companies that charge for the service.
ANI is generally not transmitted when a call is operator assisted, only the area code of the last switch to route the call is sent.
Automatic number announcement 
Early in its history, ANI was used to provide automatic number announcement, an internal test service for telephone maintenance workers. The service was generally unknown to the public but could be accessed by dialing special numbers reserved for testing purposes.
A related service feature available to Inward WATS subscribers is the Dialed Number Identification Service (DNIS), a code related to the number that the caller dialed when accessing the WATS line. With the information, the service provider can have several toll-free numbers directed to the same call center and provide unique service based on the number dialed. DNIS can also be used to identify other call routing information. For example, the WATS service can be configured to send a specific DNIS number that is assigned to callers from geographic regions based on city, area code, state, or country.
Similar services 
- Europe: Calling Line Identification (CLI)
- United Kingdom: Caller Line Identification (CLID)
- Australia: Automatic number identification (ANI) 1800801920 or 12722123 (Telstra line only, local call cost)
- US Patent 2,265,844: Calling Line Identification Circuit
- US Patent 2,300,829: Calling Line Identification System
- "ANI II Digits". North American Numbering Plan Administration. 2003. Archived from the original on 18 June 2007. Retrieved 2007-07-06.
- See Gizmodo