Iphone call volume too low

I recently bought a new Iphone 4 but have experienced a problem with very low call volume, I can not hear the other person on the line at all if there is any background noise. I have removed all packaging including the plastic film so this cannot be the problem as it has been in some cases. I was able to have a hour long phone call to begin with and everything worked well and after this the volume just dropped.

I have fiddled with the in-call volume buttons but it seems to make little difference.

Thanks for any help, I'm sure there is something really silly that I'm not doing to fix it. 🙂