Two-way transmission circuits that typically directly connect a PBX in one location to a PBX in one location to a PBX in another. Tie lines are normally arranged for two-way calling. Calls from an extension at one location can be placed to an extension at the distant location by dialing a short access number. In most cases, this type of circuit is terminated with a four-wire analog local loop on both ends and uses MF signaling with E & M supervision. Tie lines can be used to support voice and/or data.