FXO (Foreign Exchange Office)The Analog Trunk FXO is an interface that connects to a trunk line, like the one from your service provider. It receives ringing voltage and battery from FXS devices. FXO interfaces are used to connect your PBX to the PSTN. One FXO channel is required for each line you receive from your telco. The Analog Trunk FXO provides connectivity to analog central office loop start trunk line analog interfaces. And it can be combined with BRI, PRI and GSM module. Typically, a phone is connected to a loop start line from the Telco and is, by default, in an on-hook state. When the phone goes off-hook, the phone completes the line circuit and current flows through the line. The Telco provides an audible dial tone to the phone and accepts dual-tone multi-frequency (DTMF) digits from the caller. The Central Office Interface Circuit (COIC) is the heart of the loop start Analog interface, and takes the place of a telephone when connecting to the Telco.The loop start Analog Interface provides the following features:- Detects ringing when on-hook- Detects loop current when off-hook- Detects tip/ring voltage crossing a set voltage when on-hook or off-hook (called DC triggering)- Can accommodate private branch exchange (PBX) with -48V battery voltage- Loop current range: 13~110mA- Voltage detection threshold is software-selectableThe IPX-21FO module applies to PLANET IPX-2100/IPX-2500, IPX-2200, IPX-2100v2/IPX-2500v2 IP PBX System. It has 4 RJ11 ports for PSTN line connection. Each port has one LED (light-emitting diode).