Mic stops working mid call

[ncwxnut]

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.