Sounds like your main microphone is faulty, or you have something stuck in the holes. On the bottom of your phone, the microphone is behind the holes next to the headphone socket. Give them a clean out and see if there is any difference. A good way to test that microphone is to record a voice memo, and play it back.
If cleaning doesn't solve your problem then you have a couple of options. You can exchange your phone at Apple. Will cost USD$269.
Alternatively, you can forgo post warranty support from Apple and find a reputable independent repairer who can replace the microphone for $50-$70.