Speak into the holes at the top and bottom of phone that's where your mic is. Or clean the area where the holes are they tend to get clogged with dust
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=== Update (07/29/2017) ===
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Microphone are the holes at the top and bottom of the phone they tend to get clogged with dust. You may use a small pen needle or blow air into the holes. This actually work with my phone which is a ZTE XL.