When people call me they can't hear me?

I have the iphone 5c and I've had it since christmas, it works perfectly but now and again I take a call and they can't hear me at all? I hear them just fine
I recently just downloaded ios7 would that make a difference?

Casey there might be 2 reason.
1. The new software may be the reason.
2. The mouth phone is not working properly.
So just take the iPhone to the nearby apple store and check it or else update the software which is compatible with the handset.
Happy calling.