I have searched high and low for an answer, the problem is absurd and has been known for about 2 years at least: the sidetone "feature" amplifies voices. So, if the wearer is in a noisy environment (THE use case for best-in-class noise-cancelling headphones), whenever one doesn't speak but tries to listen to the call, the surrounding background noise gets amplified. Result: Terrible disturbance of the call.
Solution: COVERING the sidetone micros with tape!?!
From now on, I suggest that all disappointed Bose QC35 owners comment accordingly in all the excellent reviews of this product. They are misleading. Effectively, these otherwise great noise-cancelling headphones CAN NOT be used for calling in noisy environments of business travelers (e.g. airport, train, street, cafe).
Re: QC35ii - Sidetone useless in noisy environment
So are we saying that for example, I would be better off in a phone conversation using my Soundsport Wired in-ear buds rather than using the QC35i if it is a noisy environment? I read the explanations but I honestly do not recollect wondering about my voice and the background noise when I have been a conversation with either of those two. In fact, the only thing I do when I am having a call in a noisy environment is the same thing I would do about a conversation in that environment - take it somewhere less noisy, if possible.
Now that I have been 'educated', I will pay a bit more attention to the next time I am in a predicament of having to make a call with one of the Bose devices where I can not move to a place that is more quiet.