I assume you have "Easy Communications" ticked off in the gameplay options menu?
First make sure you have the correct frequency for the FARP from the mission planner (click the T symbol, should show the frequency.)
Next, make sure your intercom dial is on the right mode. If i remember correctly, 2 is for UHF radio and 3 for VHF AM.
Third (and is where I think the problem is coming from) you have to use RAlt + \ to use the cyclic radio trigger for the radio to work correctly.
Hope this helps!