One type of off-premises extension, connected to a private branch exchange (PBX), is generally used to provide employees with access to a company telephone system while they are out of the office.
Another type of off-premises extension, connected to a public telephone exchange, is generally used to allow a private phone line to ring at a second location.
For example, the owner of a business may have an OPX for their home phone at the business location, allowing them to avoid missing calls to the home phone.
Telephone service providers charge a significant monthly rate for an OPX, partly calculated by the distance; in extreme cases, the distance may result in a rate higher than simply having an additional central office line with its own number.
Recent innovations such as call forwarding-no answer or simultaneous ringing of multiple lines can replace several of the conveniences of an OPX at much lower cost.