A dude is being put on blast for leaving his phone number on his restaurant receipt... and not a tip. Ok let's go over the rules, just for good measure.
1) Always tip 20%. Always. "but what if they-" yes, always. Your job doesn't dock your pay for having a bad day. They're providing you a service. Pay them. Or eat at home.
2) Don't hit on people while they're working. Ever. In any job at any time. "but my grandparents-" no one cares. You're not them. When people are working, they're literally being paid to be nice to you. It's not love. You're not hitting it off. It's a business exchange. Be professional.
3) If you get a number on your receipt and YOU are the server, check the tip. Ignore them if it's good. If it sucks, we use the robocall signup list. God speed.