Audio Quality Issue (for caller)

[Fuzzbuzz44]

Hi, my partner has the Samsung Galaxy S2 but anybody who tries to call him can't hear him speak clearly - the sound is kind of muffled/distorted. Not quite sure what the problem is but wondered if anybody has had the same problem before he takes it back to the shop and knows of a way to get rid of the noise? (PS: It's not that the volume is too loud)
Thanks for your help