An FXS VoIP Gateway connects the two systems by changing the signals. It gets voice signals from PBX lines or analog phones. Then it changes these impulses into digital VoIP packets. These packets go over IP networks to get to the phone or system at the other end. The device can also handle VoIP calls that come in. It changes the digital packets into analog signals that regular phones... https://www.akom.in/blog/explain-the-voip-gateway-and-its-working-a-complete-guide-by-redstone