What is Automatic Number Identification or ANI for short.

It is a system whereby you can learn the phone number of the person that is calling you. They can be linked to a database to find the person’s name and address.

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

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

ANI is different, conceptually and technically, then 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.
Placing a call through an outbound-only VoIP service or some calling cards will cause a non-working number to be sent as the ANI.