A feature of the public switched telephone network (PSTN) that transmits the originating telephone number in advance of connecting the call. ANI is used by telephone companies for tracking and billing purposes. PBXs equipped with proper software and with special trunking arrangements (all at additional cost) can appear to the network as a central office (CO) switch and, therefore, can gain access to ANI information. This approach offers advantages compared to calling line identification (CLID), which the callers can block, and is often used by incoming call centers. ANI is available to interexchange carriers (IXCs) subscribing to Feature Group D (FGD) termination. See also CLID, CO, FG, IXC, and PSTN.