One of the hot new topics in communications today is VoIP, which stands for Voice over Internet Protocol. Basically, you can use the Internet to make a phone call, meaning that you pay nothing but your access fee, whatever that might be. To make a VoIP call, you must use a device that serves as a gateway between your phone and the Internet. This gateway is commonly a computer, but it can also be an FXS.
FXS stands for Foreign Exchange Station, and it's the New Communication Age version of a telephone switchboard. It's not so obvious now that most calls are direct dial; in the old days, you dialed in to an operator and told him or her what number you wanted to be connected to; the operator would then dial the number and connect you to your intended chat partner. You may not realize it in the modern age, but phone calls still take place that way; today, however, the switchboard is an automated one.
An FXS is a physical device, a telephone interface, that provides battery power, a dial tone, and a ring. By necessity, it must connect to an FXO device. FXO stands for Foreign Exchange Office. In a conversation with the operator, you are accessing the FXS and he or she is accessing the FXO.
In order to make a call, you must connect an FXS device to an FXO device. If you pick up an analog phone and dial, then this takes place automatically. If you make a VoIP call, then the process takes place automatically as well, although it uses different software and functionality to do so.
Most VoIP methods are handled by software these days, and this software includes FXS functionality. You access your computer or your mobile phone using software with built-in FXS, and your call is routed to your conversation target via an Internet-based FXO. In the brave new world of VoIP, we are using new technology to do the same old thing.