iPhone 5 Speakerphone problem

Hi,

Having a problem with the speakerphone on my iPhone 5. When I phone someone and have a conversation if putting it on speakerphone the person on the other end of the phone will hear an echo when they speak, only happens when on speakerphone. Anyone else experiencing this problem or know of a possible fix?

I'm on the Orange UK network and don't have the plastic covers on the phone. I've tried it both with and without the case on.