Automatic number identification

Automatic number identification (ANI) is a feature of a telecommunications network for automatically determining the origination telephone number on toll calls for billing purposes.

In telephony, ANI service was originally created by AT&T Corporation for internal long distance charging purposes, eliminating the need for telephone operators to manually request the number of the calling party for a toll call.

Modern ANI has two components: information digits, which identify the class of service, and the calling party billing telephone number.

ANI is not related to newer caller ID services such as call display.

Automatic number identification is also used to describe the functions of two-way radio selective calling that identify the transmitting user.

Modern toll-free telephone numbers, which generate itemized billing of all calls received instead of relying on the special fixed-rate trunks of the Bell System's original Inward WATS service, depend on ANI to track inbound calls to numbers in special area codes such as +1-800, 888, 877, 866, 855, 844 with 833 and 822 reserved for future toll free use (United States and Canada), 1800 (Australia) or 0800 and 0808 (United Kingdom).

ANI is different, conceptually and technically, from 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 subscribers and large companies normally have access to ANI, 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.

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