A 4 Port FXS Gateway is a must in this situation since it serves as a link between your current analog equipment and the secure VoIP system. It is the port that gives a typical analog phone its "dial tone" and ringing voltage. Imagine it as the wall jack that your regular phone connects to https://gsmgateway972.wixsite.com/gsmgateway/post/what-does-4-port-fxs-gateway-mean