I give it at CVS because I never have my card. Ditto for Borders discount club.
Supermarkets don't ask.
Most of the rest of my shopping is online. Probably I have given those businesses my phone number, in the normal course of ordering.
I figure, my phone is for MY convenience. I choose not to answer it, most of the time.
When I forget my card and they ask for my number to look it up, I give it to them, but it doesn't work so I have to give them my cell and that still doesn't work, and then I say 'oh I must have given a fake number when I filled out my card". I usually get a look with that


Why? Years ago, (long story short) I met a guy at a store who was handsome and flirtatious, but gave me the willies. Because I was filling out a store form with him, I had to give my info. He kept commando flirting, despite many reminders I was married. Finally, I had to get snippy with him and he went from Dr. Jekyll to Mr. Hyde in record time.
After that, whenever I went in that store, he would shadow me. Gave me the creeps.
He often met women via his job and lo and behold, he had my info. He had even lived in my neighborhood at that time. Worst of all, he had a specific ""ideal woman" and I fit it to a T. I may not be every man's dream, but I was his. 


)