I have done a fair amount of researching, and this problem definitely exists, and not in very small numbers. I experience the exact same issue. Others can't hear when I am speaking, (at times for up to 5 seconds), but it does "cut back in" after a bit. There are many theories at this point, with many pointing to the metal sides, but I have no idea what the problem may be. T-mobile says they have gotten calls with the same complaints, but they didn't have any real answers on how to fix the problem. They suggested waiting for the first update to be available. Some suggest turning off google voice, "OK Google" to eliminate that as a possibility of being the problem. If the first patch from Tmobile doesn't fix the problem, I will use Jump to get a different phone, as this issue negates all the positives about this phone. :-(
P.S. Another suggestion is to make sure "Noise Suppresion" is not activated.