This is when some dick has his cell phone in his back pocket or just placed on the sofa and either they sat on it or someone else sat on it hitting the speed dialer to your cell phone. Of course they have no idea they called your number but you get to listen to all their BS.
Ever have your cell phone ring around 2:00a.m. you pick it up then answer, hello, hello, hello...all you hear is a bunch of people talking and perhaps loud music in the background. You think you recognize one or two of the voices but then you hear your name and they are saying some pretty rude things about you. Well then you've been "butt called".
A butt call is when some clueless moron keeps their cell phone in their back pocket without locking the key pad. They sit on the phone, thereby using their buttocks to dial YOUR cell number.
Ever have your phone ring at 12:30 am and all you hear is loud music and unintelligible voices - some of which may or may not be those of someone you know? Then you have been butt called.
When one's pressure on one's phone, secondary to sitting on it, calls someone, while the owner of said phone is oblivious. Not to be confused with "booty call", an entirely different situation, in most cases.