An analog telephone adapter (ATA) or FXS gateway, is a device for connecting traditional analog telephones, fax machines, and similar customer-premises devices to a digital telephone system or a [[Voice Over IP (VOIP)]] telephony network.^[[Analog telephone adapter - Wikipedia](https://en.wikipedia.org/wiki/Analog_telephone_adapter)] **FXS** and **FXO** are the names of the ports used by analog phone lines (also known as [[Plain Old Telephone Service (POTS)]]) or phones. Foreign Exchange Subscriber (FXS) is the port that actually delivers the analog line to the subscriber. In other words, it is the “plug in the wall” that delivers a dial tone, battery current and ring voltage. This is the jack or interface to the phone system which FXO devices can be connected to. FXO (Foreign Exchange Office) is the port that receives the analog line. It is the plug on the phone or fax machine or the plug(s) on your analog phone system. It delivers an on-hook / off-hook indication (loop closure). Since the FXO port is attached to a device, such as a fax or a phone, the device is often called the “FXO device.” This port establishes the connection to the analog line (FXS). FXO and FXS are always paired, i.e similar to a male / female plug. If no phone system is used the telephone (FXO) is directly connected to the FXS port. The port is provided by the telephone company.^[[What are FXS and FXO •• Analog ports | 3CX](https://www.3cx.com/pbx/fxs-fxo/)]