I am not sure if your issue is specific to your device, or if it's related to general phone usage. Sometimes, when changing phones, I have to change how I hold the new phone.
Locate the microphone at the bottom of your phone. It's a small pin-sized hole. While talking, make sure the microphone is pointed in the general direction of your mouth, and is not covered by a finger. I use my right ear, so my pinkie is less likely to cover the microphone. Finally, I have found that if you hold just the bottom half of your phone, the palm of your hand helps direct your voice into the microphone for a clearer transmission.