A mobile phone call placed accidentally and unintentionally, usually from the Send button getting pushed during normal movement while the phone is in your back pocket (next to your butt).
"My phone rang about five timesin a row and there was nobody there. Each one was a buttcall from Tricia!"
phone rings
Person 1: hello?
Persons 2, 3, 4, and 86: unintelligible drunken noise
Person 1: Is my reception bad?
Person 86: Shit, it's person 1. You just buttdialed him. Now he knows everything!
Person 1: WHAT?! HELLO?!
dial tone