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It sounds more like a hardware issue, but for now, lets troubleshoot it from the software input side.
Is your in call volume all the way up? During a phone call, push the up volume button to increase the sound from your speaker.
Have you investigated all of your phone settings? under Phone, hit your menu button and select Call Settings. There should be options like Extra Volume or Noise reduction in them. Investigate these to see which settings work best for you. Every phone is different.
Still, yours sounds like a microphone issue. Have you check the microphone port on your device to ensure it isn't blocked? Many times, an aftermarket phone case will cover up a microphone port. Or maybe there is a piece of fluff or dirt in it, causing a blockage?
Please confirm all of the above.
