ANI is short for Automatic Number Identification. This is a system used by telephone companies to determine the directory number (DN) of a subscriber currently making a call.

ANI can be likened to caller ID as both systems provide similar functions. However, ANI makes use of more advanced technologies to perform its processes. ANI uses multi-frequency signaling to transmit in-band data. ANI data can be transferred separately using an ISDN PRI (Integrated Services Digital Network–Primary Rate Interface). On the other hand, ANI is virtually impossible to block. This is in contrast to caller ID, which can be blocked by entering a set of numbers before dialing the telephone number one would call.

An ANI message consists of several pieces of information. It consists of the key pulse(KP), the information digit, the directory number, and the start signal (ST). The information digit can have a number of possible values. Automatic identification is represented by 0, while operator identification is signified by 1. Identification failure is denoted by 2.

During the early years of ANI, it was used by telephone companies to determine the exact bill they would charge to their subscribers. This ensured that the payments being given were accurate and all telephone services acquired were registered. Later on, certain telephone companies started to make ANI available to commercial customers requiring its features. One of the most prominent services where ANI is used is the 911 emergency number. Through this core technology, 911 staff can quickly determine the number of the phone used by individuals who need urgent help.