4 people in bed, places telephone on nude rear end of one person, what's the film?

This would be late '60s or maybe very early '70s. My parents went to see a movie, and I was young enough to get in even though it was probably rated R or M, possibly GP or PG (depends on what year it really was). I was young enough to remember almost nothing about it, other than a scene were 4 people were in bed, partially or totally nude, the phone rings, one person stirs enough to answer it, then picks up the entire telephone base and sets it on the bare rear end of another person while he (I think it was a he) talks on the phone.