Well, what is the MP-104???
Or what type of ports does it have???
FXS ??? FXO??? 4 wires with E and M?
Assuming that the box have the normal FXS ports, menaing that there is a port where you normally will plug a phone , You could connect those ports to a standar C.O. trunk card on the PBX.
That is the type of ports used by your everyday,normal phone lines coming from your friendly phone company.
Once that is done, you need to program the PBX to access those lines, either by dialing a access code (dial 3333 for long distance), or if you have a little key system, you may make a “hotkey” that will give you acccess to the dial tone from the box (press the second buttom on the right to call long distance)
If you get more details, like what kind of interface does the box have, and the brand and model of the PBX, I could pass mre information .
Things that do not work:(try them at your own risk.
1)Extension line of your PBX,conected to the FXS port of your box
2)C.O. trunk of the PBX conected to FXo ports on the box
one of the devices need to be “voltage provider” (FXS) and the other is a “voltage sinker” (FXO), or it will not work.