The Telex 850 takes its ANR power from the microphone circuit and works all the time with intercom on or off. As long as the mic lead is plugged in (or mic wire powered in the Airbus single socket) you have ANR. The only time I ever lose ANR is if I use the hand mic for a PA.
They're very light and take up very little space compared to something like the Bose headsets. I wouldn't use one if flying something other than a jet because the passive noise reduction isn't brilliant but I can recommend them otherwise; reasonably priced, ANR is fine for a jet, light and the ear cups are comfortable with no clamping.