Why can't I hear anything when I call someone? (iPhone 3GS)?

I still have an iPhone 3GS and recently I have found that whenever I make a call or receive a call, I can't hear the person on the other end, but they can hear me. It doesn't work when it's on speaker or headphones either. But whenever I listen to music or sound from an app I'm hear it fine. I have tried a hard resets and I don't know what to do.
How can I fix this?

Your phone is a bit and starting not to work
You may need to get a new one soon

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