Looking for wired headset options to use with PTT Express and MC75A0. I'm told that RC51 works, but we need something like part number 21-154925-01R for Retail environment. I tried 21-154925-01R which works with MC40 and SB1, but it doesn't work with MC75A0. Audio won't come through headset. It only comes through MC75A0 speaker.