IPhone 4: Callers can't hear me but I can hear them perfectly

The other person is constantly saying "hello" "hello?" "hello" while I can hear the other person just fine and talking regularly. There's a slim chance of when it actually works, and it's only for about a minute. At first I thought it was my service provider, which I called them and they suggested that I change my sim card. And I did. The problem seemed to go away for about 3 days but soon after it was just like before. I've tested the microphone by recording voice memos and they sound normal. Face timing (and face time audio) also works, but for some reason calling a number won't. I have also reset it to factory settings which didn't do anything either.

I've dropped my phone a couple times in the past few months, could something have moved within the phone that makes it not possible for callers to hear me? I just wanted to get a second opinion before I take it to genius bar.