Frankly, public places such as airports are quite obviously wrong for a very confidential call for every possible reason. Definitely nowhere in the lounges would meet that requirement (ex-boardrooom included). I agree with others that your best bet for a "relatively" confidential environment airside would be an empty gate area but no guarantee noone will come, a shower cubicle, but those are not very well insulated. If that is an optiion though, personally, I would suggest landside, outside of departures, at the very beginning, well before the first entrance, ie towards where cars arrive to drop someone and before the first batch of smokers, you'd probably be in the most confidential area T5 has to offer, plus being outside, there are more parasitic noises which would make it harder for any random passerby to overhear your conversation compared to indoors.