Mic stops working mid call

Does any one have advice on what could be causing the mic to cut out during phone calls? I am having this issue on more than half my phone calls. Only way to get mic to work again is to reboot phone. I have been on two different gnex phones and have replaced the SIM card. I want to keep this phone because other than this I love the phone but need to have this fixed before the Jan 15 deadline for returns.

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