IPhone 5 volume very low? Can't hear calls?

When the phone rings, you can hear it very clearly.same goes for message tones etc.
but when it comes to trying to talk to someone on the phone, I can hardly hear a thing. The volume is very low. I've tried putting the volume up but it already says it's on the highest volume.what's the problem here?
nothing's covering the speaker.

Have you tried cleaning out your ears?