Saad,
You can make calls between phones connected to the FXS port of a Tenor using the intercom feature. You may not connect phones to the FXO port of a Tenor as the FXO port can only connect to a phone line from phone company or from station side port of a PBX.
Basically, the FXS port of a Tenor gives dialtone, like a phone needs to work. The FXO port of a Tenor receives dialtone.
Mike