It is sounding like an RF breakthrough, most modern radios have a TX interlock function so that a pin on box 2 is conected to the MIC Key on box 1 (and vice versa).
When the MIC key is activated the pin on the opposite box being pulled to earth turns up the squelch on that box preventing audio breakthrough.
All that having been said the MX170 is not a box with a good reputation, the audio panel is getting on a bit and the sigtronics intercom is very subject to breakthrough if all the screened lines are not properly installed so due to the age of the equpment and wiring you may have to live with the problem or spend some money on new equipment.