Is it really necessary for the cashier to asks for your phone number when you purchase something from a store?

Here is my rebuttals:

#1
Cashier: Can I have your number, please?
Me: Umm… I’m quite flattered, but I’m married.

#2 (This one would probably get you in trouble)
Cashier: Can I have your number please?
Me: I’ll give you my number if You will give me your bra size…

Just sayin’…