It would depend on her tone/body language as she's saying it (and of course how the interaction went up to that point). My first inclination is to think she's just joking - a little playfulness is something I look for in women! But she would have to be smiling/giggling, etc for me to really be sure of her intentions. In this case, I might say something like "Oh man, that's unfortunate, get with the times! Well then, what's your AOL IM address?" Or even, "Oh really, what's that *points to phone*. The new Angry Birds gaming system?" You get the idea, be playful and keep pressing a bit more. If she wants to give you her phone number, it shouldn't take too long.
If she's still holding out, or maybe told you the original statement in a disrespectful tone, I'd just laugh a bit to myself (because the situation itself is truly funny! Look at the humorous side of it - if it is a rejection, mocking though it may be, it's kind of funny and clever!) and say "Ah, I see *smiling*. Well, take care/see you around."
Remember, if she wants to give you her number, she won't put up too much, if any resistance to your question. As always, we're looking for high interest. Exit stage left if she seems disinterested! However, I would almost always press a bit further regardless of what her body language was (simply because there's just so little downside in doing so). Until it becomes relatively clear that she's not interested, I would keep going.