FXS and FXO are the ports of electronic device which is used by POTS, often as an analog phone line or old telephone service. The two ports are constantly connected together in order to placing a call.

• FXS (Foreign Exchange Subscriber) port is used to transmit the internal connection to office device such analogue fax machines and analog phones. To provide dial tone, ringing voltage, and battery power to FXO devices through a wall jack or FXS interface.
• The FXO stands for Foreign exchange office that is connected to an external phone line via FXO port. On the desk phone, analog fax machine, or modem, there is a plug called an FXO port. As a result, the FXO interface receives the POTS service from the PSTN Central Office and creates a loop closure.

The FXS and FXO ports must be linked in order to make a call. Both incoming and outgoing calls are affected by this. You dial a number and use the PSTN service to establish a connection to the other end when you pick up an analog phone (an FXO device) that is linked to an FXS port. When you use your office phone to call a number, the DTMF (Dual-Tone Multi-Frequency) digits are encoded and sent to an FXS port.

The connected FXO device will begin ringing when an incoming call is received over the FXS interface, which will also send the ringing voltage. Line establishment occurs once the phone is picked up. On the other hand, communication will not work when two FXO or two FXS ports are connected. No dial tone will be delivered, even if you desire to connect two internal FXO devices.

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The CM300G analog Gateway makes it simple for businesses to upgrade their legacy PBX equipment with new VoIP access and applications. It enables a well-planned, gradual migration to an IP network. The CM300G Gateway connects to a PBX or LAN and transforms analogue PSTN messages into a format appropriate for transmission over standard IP networks.