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Terms FXS, FXO, POTS, PBX explanation

When a customer receives phone service from a central office other than the one that would normally serve them, the line between the customer and the "Foreign" office is called a "Foreign Exchange" line. FXO (Foreign eXchange Office) is the office end of the line, and FXS (Foreign eXchange Station) is the station end. FXO and FXS are also used to refer to the type of interface on phone equipment.
- An FXS interface provides power (battery) and generates ring signals.
- An FXO interface receives power (battery) and receives ring signals.

If you want to connect an ordinary telephone to a computer, you need a card in the computer with an FXS interface. If you want to connect a line to your computer so that it can make and answer calls on your analog PSTN telephone line, you need to add an FXO interface to your computer. Always check the sense of the device whether it is labeled FXS or FXO, as some vendors get it backwards and your device will provide power when you expect it to behave as a station and receive power.

POTS - what it is? Analog telephony, also known as Plain Old Telephone Service (POTS), is the service the local phone company typically delivers to your home. Local phone companies deliver POTS from their Central Office (CO) to the subscriber’s premises over a circuit consisting of two copper wires. To increase the distance over which the signal can be transmitted the two wires are twisted together, which also reduces electromagnetic interference. So these two-wire copper cables are commonly known as “twisted pairs.”

FXS - Foreign eXchange Subscriber interface (the plug on the wall) delivers POTS service from the local phone company’s Central Office (CO) and must be connected to subscriber equipment (telephones, modems, and fax machines). In other words an FXS interface points to the subscriber. An FXS interface provides the following primary services to a subscriber device: Dial Tone, Battery Current, Ring Voltage.
You may also see the FXS acronym rendered as Foreign eXchange System.

FXO - Foreign eXchange Office interface (the plug on the phone) receives POTS service, typically from a Central Office of the Public Switched Telephone Network (PSTN). In other words an FXO interface points to the Telco office. An FXO interface provides the following primary service to the Telco network device: on-hook/off-hook indication (loop closure).

PBX Connections and terms FXS/FXO

A PBX provides both FXS and FXO interfaces. FXS - When you connect a PBX to analog phones, you plug phone cables into FXS ports on the
PBX. The FXS ports on the PBX provide POTS service, including battery current, ring voltage, and dial tone to the phones.
FXO - When you connect a PBX to the Telco Central Office, you plug the (FXS) lines from the phone company into FXO ports on the PBX. The FXO ports on the PBX provide on- hook/off-hook indication (loop closure) to the local Telco network.