Immediately following the SLI I took a bid from IAD to EWR and was one of the earliest UAL pilots to fly with an all CAL crew on a CAL airplane. When I showed up at the plane there was no mini-tel and I had to ask maintenance to bring one up. Subsequently I learned that at CAL pilots purchased their own headset. I bought a headset for myself ( Telex Airman 8) and have worn one ever since. I much prefer sharing my own kudeees on the mic then the kudeees of other UAL pilots
but seriously I agree we should all use headsets. Trouble is our FOM is still beholden to "Gold Standards" UAL acquired from the Pan Am era. I remember one FO complaining about a UAL legacy captain who is very high in ALPA even today who would berate her for wanting to use the intercom and explain how her use of a headset was dangerous as it reduced her awareness of ambient noise. That's the heart of the problem as regards headsets - old dudes from my generation who seemingly have a hard time learning new tricks
and don't even get me started on cross tuning in the triple which some silly UAL pilots still forbid - unbelievable!
FOM 17.10.1
A personal headset may be used on the flight deck for communication only if
approved under an FAA Technical Standard Order (TSO) for aviation use. If the
headset is of a noise-cancelling type, it will reduce background noise levels that
can diminish the volume of important environmental sounds and alarms. The
FAA cautions that these headsets may make it difficult to hear communications
between pilots, abnormal engine or mechanical sounds, aural alarms,
vibrations, and wind noises. Pilots using TSO-compliant noise-cancelling
headsets should perform an evaluation both on the ground and in flight during
normal operations to ensure their model of noise-cancelling headset allows
them to hear the sounds and alarms described above. If the pilot's model of
headset does not permit the sounds described above, it shall not be used.