Electret is a comparatively new invention, and is another name for "condenser" or "capacitor" microphones. Aircraft intercoms and radios predate electret by many decades.
The difference in aircraft systems is between dynamic (moving coil etc) microphones, which don't need powering to make them work but which need amplifiers after them, and carbon microphones (as fitted in domestic telephones in the 1940s and 50s) which need a polarising voltage but don't need an amplifier.
Old headsets as used by PPLs tended to use carbon microphones, so the aircraft had a power supply to feed them. When electret were invented, they needed a polarising voltage and an amplifier to make them work, so it was easy to adapt them to old aircraft systems.
Before that, there were aircraft headsets with dynamic microphones and amplifiers, the latter powered from the carbon mike polarising voltage.
That's the history (more than you needed!).
The catch, nowadays, is that not all microphone systems (electret, dynamic, or carbon) are the same impedance. If your aircraft simply connects all the micophones in parallel, then only the lowest-impedance one is likely to work properly.
Some DCs won't work with some other DCs, for example. There are ways to fix the problem if it occurs, but a decent intercom unit is the best solution.
For more information, consult Richard Holder, who will also for a very reasonable fee convert your non-ANR headset to ANR. He has done much research on microphone compatibility.