iPhone 6 Plus muffled sound

Recently my iPhone 6 Plus seems to have developed a problem where the sound received when making a telephone call is muffled almost to the point of not being able to hear it. The person on the other end says I sound fine however.

Any ideas what the problem could be? Has some dirt got lodged somewhere (I can't see anything obvious). Do I have a 1 year or 2 year guarantee being in the UK?

Thanks