IPhone 3GS: How can I make the phone volume louder?

I have the phone volume at the loudest level, but it's still very hard to hear the person I'm talking with sometimes. Is there any way to make it louder? Not on speakerphone, but on the small speaker you put your ear against during a call.

I don't know

Why don't you upgrade to a iPhone 5. You could get a Unlocked/AT&T iPhone for cheap on eBay.

by the way, i don't really know how.