An FXS (Foreign Exchange Station) gateway is a device that connects analog devices, like phones and fax machines, to a VoIP (Voice over Internet Protocol) network. It acts as a bridge, converting analog signals into digital signals for transmission over the internet. Essentially, it allows businesses to use their existing analog infrastructure while transitioning to VoIP technology. FXS gateways convert the analog signals from traditional telephones, fax machines, and legacy PBXs into digital signals that can be transmitted over the internet.