Automatic number identification

What is automatic number identification?
Automatic number identification (ANI) is a feature that enables a telephone to display the number of the calling party. How this information is provided is determined by the service provider. This allows users to review their calls. ANI is also used by emergency call centers.

ANI was created by AT&T for internal remote billing. It's separate from Caller ID, a newer service that performs a similar role but uses different underlying technologies. In the United States, ANI is part of the Wide Area Telephone Service (WATS).

ANI describes the two-way selective calling functions that identify sending users. Customers buy WATS because they are taking advantage of a way for prospects to call the toll-free number. Customers are provided with a separate toll-free phone with numbers that begin with special area codes. The participants of these numbers are referred to as Inward WATS participants.
Unlike Caller ID, ANI is block resistant and is offered to commercial customers who want to know the numbers for the calls they are receiving.

The ANI service is provided by sending a digital multi-frequency tone along with a call. The data burst has a unique number with a length of one to eight digits. A caller's phone number is captured by an ANI service even if Caller ID Blocking is enabled on it. The destination telephone company switch provides the ANI delivery service subscribers with the telephone number of the caller. Residential customers can also access ANI data through some third party companies that charge for the service. Calls via VoIP service and calling cards send a working number as an ANI.