What is..

.. the best way to refuse to give away your cell number to a guy? My room mates this evening were very furiously trying to come up with an answer to this seemingly frivolous question. If some guy at a party asks you for your number and you dont want to give it away, what is the nicest way to refuse? We had no clue. We googled, we yahood, we even asked Jeeves but the solution still evaded us. So the internet does NOT have answers to all our burning questions. Apparently many people have asked the same question. So, what is it? Please email us if you have some useful advice. Please.