I used a little Panasonic cassette recorder which has a vox setting, plus a little mic on a long cord.
Put the mic inside the earcup, put the recorder on your belt, and adjust the vox sensitivity until it only breaks when you press the intercom button or the radio receives something. The recorder stops after 10 secs of silence.
The end result is a faithful record of all intercom and radio comms, without the huge gaps of silence that come in real time. great for giving the cassette to the student after his flight, to listen in the car on the way home. Amazing how many times they hear me telling them to do something before they actually do it.