PTT = Push To Talk, it's what the pilot use to transmit his voice, and like in the A-10 where you have 4 radios (UHF, VHF AM, VHF FM and Intercom, you'll have 4 PTT buttons. If you don't use the one corresponding to the radio you want to use, you'll get no response. I'm guessing it's like that for Huey as well.