Nokia :: X6 - Phone Rings But When I Unlock The Keyguard The Phone Stops Making The Ringing Noise

I have the X6. The phone rings and I unlock the keyguard using the slider on the side of the phone and the phone stops ringing. Then in order to answer the phone I press the green button on the bottom left of the phone or I press answer call on the screen and then I can speak which I suppose sounds fair enough. But I just think that something is wrong because surely the phone should continue to make the 'ringing' noise until I actually answer the call rather than going mute just because I have unlocked the keyguard.. Am I doing something wrong or is this just how the phone works?