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. Automatic number identification 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. Toll-free numbers. 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). Privacy. 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. 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. ANI is also not supported properly for calls originated from four-party lines. Spoofing. For quite some time it was thought not possible for someone to spoof ANI. While it is not an easy task it is doable. Automatic number announcement. ANI is used to provide automatic number announcement, an internal test facility for telephone maintenance workers. The service, which is not advertised to the public, allows an installer to  identify an individual line by using it to dial a special unpublished number in a range reserved for testing purposes (such as 958-xxxx in much of North America). DNIS. A related service feature available to private branch exchange subscribers is the Dialed Number Identification Service (DNIS), a code indicating the number requested by the caller. With the information, a 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, toll-free 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.