Interesting subject. I remember when EGNM had its own home-built RDCS and TDCS and I had the opportunity to listen in a few times. There was certainly RTF sidetone fed to the headset - I don't know the source. The controller would usually have up to three RTF RX frequencies selected - whichever was also on TX gave the loudest volume which I guess was deliberate. I remember the volume on the external telephone lines was on the low side but the internal intercom between control positions was much louder - it was a simple ring-main system connecting all positions. Now it's a Frequentis VOIP system. I don't know how it compares.