I had two similar issues. Though not totally the same as described here, it might be a start of a collection of possible problems in this scenario:
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For one, the microphone in the bottom module could be damaged; for me, it stopped working altogether at one point. To debug problems with the microphone module, the soundrecorder is a good starting point, as already mentioned. Also, if call quality is fine while working over the “louder” speaker (which uses a different microphone), this is a good indicator that there is indeed something wrong with the microphone in the bottom module; in that case, I would recommend to contact support.
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If the call quality is only sometimes bad: I seem to have a reception problem, but a one-directional one: The symptom is that when I’m in such medium to low reception areas, while I am on a call, I can hear people just fine, but they can’t hear me (or only very intermittently). See also a more detailed description here: Bad outgoing signal / call partners don’t hear me or only interruptedly . I have no solution for this yet; the only thing that helps is to go to some place with better reception.