Rather than argue, I always just gave an old number that I no longer used. I still make up dates of birth when they require one, unless it's an official thing. I make up email addresses, too, if they ask just to give me a shipping cost. That is none of their business, especially with identity theft.
Reminds me of my friend who, as a child, told her brother he was adopted. He asked his mother, who told my friend to stop telling him stories. My friend retaliated by asking him if he thought his mother would tell him the truth!