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An off-premises extension (OPX), sometimes also known as off-premises station (OPS), is an extension telephone at a location distant from its servicing private branch exchange (PBX). An off-line extension is generally used to provide employees with access to a company phone system while they are out of the office.
OPXs are used in distributed environments, serving locations that are too far from the PBX to be served by on-premises wiring.
An OPX uses a conditioned wire pair that is usually used only for voice applications, while for data, a pair usually needs to be unconditioned. An alarm circuit is an unconditioned pair.
In Internet telephony, a VoIP VPN OPX may be implemented by connecting an extension over a virtual private networking connection, instead of connecting it directly to the local area network. As the VPN already appears virtually to be part of the local area network, the off-premises extension appears to the IP-PBX as if it were on-site.