I don't know if this has been done before. I got the idea after watching Season 1 of The Pick-Up Artist, when Mystery said to number close by giving a girl a pad of paper and saying "I'm leaving soon, show me something creative" or some such.

You need something to write on for this. Draw a hang man gallows, and underneath:

(_ _ _) _ _ _ - _ _ _ _

Segue into this by saying something like, "I'm leaving soon, and I could just ask for your number, but it'd be so much more fun to guess it instead." Or alternatively, "So you don't/do believe in ESP? Well I bet I can guess your phone number."

Then play as you normally would, but with digits instead of letters.

But here's the best part - I've tried this with a few numbers on my phone, and for this to fail, her number would have to be made up of four or fewer distinct digits (as you get six number guesses before you're out). That's extremely uncommon, and the probability is low that you won't hit any of the digits before you're out of guesses. Correctly guess four of the digits at any point in the game, and your win is guaranteed. I haven't run the probabilities, but even if after you get two right, the likelihood of losing is very low (as you would have 8 digit guesses total).

I haven't tried this out in the field yet. But if you need to better your odds, know the local area code - if you get that right, you're pretty much guaranteed a win (9 guesses total, at minimum).

I've done this...
Usually I do it two times.. The first time I would type:
(_ _ _) _ - _ _ - _ _ - _ _

Usually if they don't get it right I would type the area code .. Like:
(915) _ - _ _ - _ _ - _ _

So now its more obvious what the point is