my incoming call volume is really low,why?

So I just got my Iphone 4s for my birthday yesterday January 8. I notice when i recieve call or make a call, i can BARLY hear the person I speak to. However my volume is turned ALL the way up, my music can be heard just fine and even when i go on youtube the volume is loud. but why can't i hear anyone when i want to use my iphone normally?

Any information would be useful. Thank you!