Squadgy
The first real clue...well done that man!!!!
Could be a classic case of controller headset mic being too far from the mouth...when you mention it...controller adjusts mic boom and all works fine.
Also some heavily used headsets/desk mic's get a little tired after many years' service and need replacing....nothing lasts forever; could be the plug, the lead, internal connections loose, desk socket etc.
Where I work there is no headset maintenance or replacement schedule, you just use the same one year in/ year out.....and if it breaks you just go to stores and get a refurbished or repaired replacement. Sometimes you get a good one back, other times you get a real turkey!
Doesn't explain the geographical positional problem referred to earlier though.