But why '?

my thought exactly

Why 39 and not 69? (A)

@ Guillaume: that's why you don't 'get' her number

If a guy ever gave me his phone number in binary I would be tickled~ geeks are adorable. :)

People only seeing the back will not see the fun and charm of it.

What's there to "understand" about that number? Is it possible to not understand a number, it's a number right? Given that there is nothing to understand about it, so no one has a clue what it's on about, does that mean everyone gets her number, is that the joke?

I would have it: "There are only 10 types of people in the world: Those who understand binary, and those who can have my phone number".

Wow... this only advertises that this chick only likes dumb guys...
Sigh...
People.

Maybe it's a challenge...

I don't understand the number. The base is not defined. Could be any of 4, 39, 256, 1045, 3156, 7819, 16864, 32841, 59140, etc. Does that mean i can get her phone number.