Some of the things can be more attractive then being flirty like being confident, playful and clever. The next time when you hit it off with a classmate, coworker or your friend of a friend slips him your digits in a sneaky way.
Here are two steps to follow for giving your phone number to a guy without being obvious:
Step 1: somehow get your hands on his phone and save your contact info to it. The main thing is to get his phone no matter how, you can grab it without knowing him is an invasion of privacy and is likely to irritate or offend him more than anything else. Entice him into showing you a picture or song, and ask for the phone to hold on your own for having a closer look on the picture or song. Work a bet or fascinating fact into the conversation, and then say, let me see your phone, I'll show you something, and dial or save your own number just before you pull up Internet search results. When you leave, tell him to give you a call sometime, and let him discover that you have given him your number.
Step 2: coax him to hand something to you and slip your number to him when you hand that item back. This trick usually works when you both are together for a while like in any party, bar or restaurant. Excuse yourself and jot your number on a piece of paper and when you come back continue your conversation as normal as you was doing. After a while ask him to handover any item, whatever is near you and when you are about to leave reiterate how nice it was to talk with him, and hand him the that item along with your number slip as you say goodbye.