I have had this problem periodically with my current Samsung S6 phone, where I am in a conversation with someone, and speaking to them, and they say 'Hello?" and can't hear me, and then after some time, they can hear me again. This will happen when I am stable in one spot, not just moving around, and in areas where there are 3 bars or more of service. During this time, I can still hear them, but they cannot hear me. It is not every call, but frequently over time.