lol...those rejection numbers are pretty funny...
If I don't want a guy to have my number, I usually either say:
1) I'm sorry, I don't feel comfortable giving out my number to people I meet at a bar/club/library
2) I would, but my boyfriend probably wouldnt like it
Or sometimes, I just give it to them and never answer the phone- the beauty of caller ID.
Regardless, saying a straight up no is kinda harsh. I imagine it takes a bit of bravery to ask for a number anyway, so I'd like to save them some immediate face in the situation.
Anyway, guys tend to think a conversation is going really well, when a girl is just trying to be nice.
(I also had a friend who use to give out the number of the local police department....but wouldn't it be weird if he recognized it?)