FXS and FXO are name of analog phone line used port (also called POTS - common old telephone service) 

FXS - peripheral exchange user telephone set interface is the transmission port of analog circuit to the telephone set. In other words, send a dial tone, battery current and voltage bell ring.

FXO-- peripheral exchange interface is the port of accepts analog line. It is a socket on a telephone or fax machine, or analog phone system, used to transmit on-hook / off-hook indication (loop closure). Due to FXO ports attached to the device, such as a fax machine or a telephone set, so these device is usually referred to as "FXO device." 

FXO and FXS are always complementary relationship, similar to the plug and socket. 

If there is no corporate communications switch, the call will be connected directly to the FXS port provided by the telephone company.

                                                   FXS / FXO without no PBX system

If you have a corporate communications switch, you can assign line provided by telephone company connected to the enterprise communications switch, and then connect the phone to the corporate communications switch. Thus, corporate communications switch must have both FXO ports (connection FXS port provided by the telephone company) and FXS ports (connection telephone or fax machine).

                                                                             FXS / FXO with PBX system

TAK’s voice over fibre multiplexer support FXS and FXO ports. Each side linked by fibre optic, It has different kinds of voice over fibre multiplexer, from supporting 1 ports FXS/FXO to maximum 360 ports FXS/FXO with Etherent,E1,Data etc.

voice over fibre multiplexer: 

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