Might vary by locale? Around here (South US) it seems like every single store has their own rewards/discount/whatever system that requires your phone number but it’s not necessary for the transaction… It’s just an extra info grab.

Sometimes the user facing POS/credit card reader will let you handle it (enter/skip) but many places rely on the salesperson to ask and then enter it or skip.

But, I also don’t get around much so my experience is limited.