If you are interested in more detailed information about FXS / FXO port working, please refer to the following specific order: 

When you want to make a call: 

1. Pick up the phone (FXO device). FXS port detects whether you have entered the hook. 

2. Dial the phone number, which will serve as a dual-tone multi-frequency (DTMF) digit is transferred to FXS port. Inbound call 

3. FXS port accept the call, and then sends a ring voltage to the attached FXO device. 

4. The phone rings 

5.hook as soon as possible in order to answer the call

End call - normally the FXS port relies on the connected FXO devices to end the call.

Note: The analog phone line transfer about 50 watts of DC power to FXS port That is why when you come into contact with the connected telephone line, you will feel slight "shock." Thus in case of power failure still can make a call. 

TAK’s related product E1 PCM multiplexer series is support 1-30 ports FXS or FXO,with Ethernet ,RS232 data,SNMP management available. Following is the one of pictures of  E1 PCM multiplexer series: