What is a polite way to turn someone down?

There has been a few guys lately who ask for my number and if I have a boyfriend etc. I don't have one, and I'm not one to lie, so I tell them I don't.. but what do I say when they ask for my phone number? I don't like giving it out etc. and I don't wanna be some kinda call lol cause that is how they make me feel.. I lost a lot of weight recently and I've never really had any attention from guys and this is not how I wanna get it.. it's always when I'm walkin my pit bull and I'm all sweaty LOL. So what do I say?