I am new to VOIP or telephony stuff & have google for difference between FXO & FXS.

We have a Assembled computer and have installed freepbx 1.8, along with it I have install TDM410 with 4 modules of FXO.

Initial installation was successful, A telephone line from Provider was connected to FXO port1 and out going call was successful.
Afterword system was taken to outstation were they don’t have direct line of provider, instead line is terminated to PBX, from PBX it is connected to my FXO, I understand but still in dilema that will it work? at present outgoing calls are not completed as dialed.

Please help me with understanding.

You didn’t tell us much, other than you installed an old version of FreePBX that is end of life and unsupported.

The understanding you need is that to connect to the foreign PBX you must properly match the interfaces. If the interface is a 2 wire then if you choose FXO/FXS signaling you must connect an FXS port from the foreign PBX to your FXO. Keep in mind the circuit could also be a tie line, wink or loop start and support E&M signalling. At the heart of any (not just Asterisk) interface project is to match the two connections. Once you do this you can tailor trunks and routers to send the correct digits and route calls.