OPX refers to off premises extension. An OPX line permits a telephone not at a company’s location to function to all intents and purposes as though it is located at the company’s location. This capability becomes particularly interesting with the recent increase in telecommuting. Suppose an employee plans to work at home. One of the problems to overcome in such a case is the isolation such a worker might experience. Providing the employee a telephone that looks like an internal line at the company might help to reduce the problem. Others calling the line within the company will dial an internal extension, which will ring at the employee’s home; if the employee wishes to make a long-distance call, he or she usually just dials y and the rest of number just as though they were at a desk at the company’s location.

As with an FX line, an OPX connects from the company’s location to the local CO, then continues via whatever intervening COs are necessary until it terminates directly on a telephone at another location. A key difference from an FX, however, is that on the PBX an OPX is connected and configured as a telephone rather than a trunk. This results in a limitation on the type of service provided: normally, only an analog telephone can be used at the end of an OPX because the digital signaling between a PBX and its proprietary telephones will probably not successfully make it through the various analog and digital circuits that make up the OPX. This limitation is not normally a show-stopper; rather, it just imposes on the telecommunications manager the need to configure the PBX to support a certain number of analog telephones as well as the digital telephones that might be used in-house.