An FXS stands for foreign exchange subscriber. FXS port is an interface device which connects station devices like phones or PBX to VoIP adapter. FXS has RJ11 port that connects internally to an analog office phone or fax machine. FXS port is connected with an analog DEVICE and the cable of the device will never leave the building.
As we discussed above, a telecommunication line from an FXO line port must be connected to FXS device so that the port after connection will work. Similarly, there is another line which comes from an FXS port and connects to FXO port for the connection to work. After when FXO port on your analog telephone is connected to the FXS port, you will receive (FXS) service and you hear a dial tone when you pick up the phone.