When a boyfriend/girlfriend calls up their significant other to talk dirty to them over the phone only to realize they've mistakenly reached their friend and/or roommate instead and hilarity ensures (see: The Terminator, Human Traffic.) There are also variants of this, such as in Bridget Jones' Diary, when she answers the phone claiming to be a "wanton sex goddess" only to figure out that her mom was calling her. Also in The Cable Guy when Chip pretends to be Robin to get Steve on the phone and he starts to dirty talk until Chip laughs and reveals it's him instead.
Hello, Unknown Troper. You'll need to get known to lend a hand here.