What is FXO?

FXO (Foreign Exchange Office) is a port used by a telephone system (typically a PBX) to connect to an analog telephone line, commonly from a public switched telephone network (PSTN). It's essentially an interface that receives the analog phone line. The device with the FXO port is the one that receives the signal, analogous to answering a phone call. It's used to terminate analog lines into VoIP systems or other telephone equipment.