It's not against the law to call the President, is it?
No, but calling him on his "super-secret," private line makes folks a bit jumpy. I watched the MSNBC segment (which I hope they post online)on television and they said that the kid hasn't told the police or the Secret Service how he figured out the telephone number.
I don't see how you can work out the number from the video. Ellen's hand is covering the keypad, and for the first few digits Jenna dials, the phone is completely out of shot: you can only see Jenna's arm. But the phone-line is being intercepted somewhere to be able to broadcast the sound into the studio, and this interception probably allows people working on the show to work out what number was dialled. Perhaps someone working on the show has a friend in Iceland?
Besides phone phreakers (I'm not sure how many of them there are these days), such a thing is also a skill among credit-card fraudsters (see, e.g., when Lester Freamon convinces Bernard of his credentials).
You could go a long way with deductive reasoning, I think, and pretty quickly narrow down the range of possibilities.
Hmmm ... maybe it wasn't that hard to figure out...
Hey, Mr. President -- what would you like your "super-secret," "double probation" telephone number to be? The line at which Cheney, your buddies in Skull & Bones (all except John Kerry), and lastly your family can reach you?