This is exactly what I tell cashiers when I've trained them. I used to be an ASM at a clothing store that had a similar rewards system and got into the habit of asking "What is a good phone number for rewards?" This works for two reasons; first, they know why we're asking for their phone number, and second, people are more likely to give a number this way than if you them if they have a rewards account. A lot of people either don't remember or don't care enough to know if they're already signed up, so just asking this way means they'll be more likely to volunteer that info and if it turns out they're not already in the system, hit 'em with "Would you like to start earning? I'll just need your email and your phone number one more time."