(Private Branch eXchange) An inhouse telephone switching system that interconnects telephone extensions to each other as well as to the outside telephone network (PSTN). A PBX enables a single-line telephone set to gain access to one of a group of pooled (shared) trunks by dialing an 8 or 9 prefix. PBXs also include functions such as least cost routing for outside calls, call forwarding, conference calling and call accounting. Modern PBXs use all-digital methods for switching, but may support both analog and digital telephones and telephone lines. See IP PBX and WPBX.