An OPX line is ordered from the ILEC when a line from a PBX is needed outside of the building where the PBX is physically located. A designed and conditioned path is created from the PBX site (A-end) to the off premise site (Z-end) The path can ride any amount of channelized T-1 interoffice carriers depending on the distance from A-Z. From A and Z offices, the circuit usually goes out to each location on copper or a SLC system. This enables the user to have the same features of the PBX in the off prem location. Such as Centrex features, 3 or 4 digit dialing to the main PBX users, transfer capabilities, etc. They usually are not restricted and the user can call anywhere and can receive from anywhere. (A ring down circuit, on the other hand, is only able to do point to point only, and only one way.) There is still plenty of them out there.
