FXO port Foreign exchange office port. Also, know as a foreign exchange office interface. FXO ports show up in many different places, including the following (1) As the RJ-11 port on a telephone, a Fax machine or modem that connects the device via an RJ-11 cable to the wall jack (which is an FXS port) or an FXS port on an analog phone adapter, which in turn has a network connection to either an IP PBX or a VoIP provider. (2) As a port on a PBX that connects the PBX to a PSTN line or to an analog telephone adapter, which in turn has a network connection to a VoIP provider.