If you think you have a hardware problem, contact support! If they can’t help you (e.g. because it is a hardware issue) you will most likely get a replacement under warranty.
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This pretty much describes my problem as well.
Proximity sensor works, the screen blackens during the call, and still … they can’t hear me anymore after a minute or so. 
So your phone does not drop calls but you can’t be heard? Have a look at this thread, maybe you find some helping answers:
Do calls on the speaker work properly? Then you might have an issue with your bottom module.
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Thank you, but not exactly. My microphone works for a minute or so and then the other person cannot hear me anymore. While the call is still active.
As I asked above: does this also happen when you put the call on the speaker?
It can be a problem with the proximity sensor. If the display is not off during the call, that can be the problem.
Have you installed latest Firmware and run Proximity Sensor calibration?
i don’t put the call to the speaker. the proximity sensor usually works, i.e. the screen turns black.
I have installed FP Open 16.12. and have run the sensor calibration, yes.
If the screen wouldn’t turn off I might be touching the “mute” button I guess.
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That may be so, but it’s a good way to check if your problems may be caused by the bottom module or not.
I had this dropping call problem too. After quite a while I actually found out that the way I call was the thing I did wrong. I always call on the left side of my head, pushing the upper right corner of the phone to my ear (don’t ask why, it’s a strage habit 
). The proximity sensor couldn’t find anything near, so the screen (and the touch screen) stays on, while touching the screen with my head (I accidentally put it in airplane mode, or call someone else) resulting in a dropped call. Now I call on the right side of my head, so the proximity sensor always has something near.
So maybe it’s the way you put the Fairphone to your head…