Why Do Department Stores Need My Phone Number?

I was at a "big box store" over the weekend and when I went to cash out the cashier asked for my phone number. Now, my phone number is my business line and I always worry about the number being added to a list of contacts to be sold to the highest bidder. I worked in telemarketing during my youth, calling people during dinner or their favorite show trying to sell them something. So, I know how the list gets to a telemarketers computer.

So, I gave her a fake phone number. She says "Peter?" and I responded YES! Thanking my lucky stars that I was paying with cash and not my credit card. She then tells me "I" have $3.17 on my account and asked if I'd like to use it. OF COURSE I declined.

But... I know it's for some rewards system that is a clever cover up to track my purchases and send me coupons for what I usually buy to keep me from going elsewhere to buy them. Still with me? Good! See I just feel that my phone number should be my private info.

Do you give the cashier your phone number or a fake one? Or do you just politely decline the question and make it uncomfortable?