FXO Gateway works in a way that connects your devices to an external telephone line and supports failure or recovery in case of an Internet crash or network failure. The FXO gateway can be implemented to provide access to multiple POTS lines; The gateway usually comes in a configuration of ports 1, 2, 4 and 8, 24, 32.

FXS (foreign exchange subscriber) gateway mainly used to connect your traditional telephones and fax machines to a VoIP phone system; FXO (foreign exchange office) gateway is used to connect your VoIP phone system to your PSTN lines.