"Butt Dials"

I really don't understand this, perhaps because I don't use a cell phone/smart phone.

"Butt dials"

Peter Moskos links to a BBC piece on the problem of accidental 911 calls.  Judging from the article, the UK has had the same problem, except their emergency number is "999".

I understand that making an emergency call is easier than a regular one, but what I don't understand is how the butt knows to dial 911 in one country and 999 in the other?  Do people have smart butts, smart enough to know the different numbers? Never knew that.