If you hear a message that sounds abusive, press pause, ask your child to leave the room and carry on listening to it. I'm not saying he should be nasty over the phone, but now that you know he can be, it's one way of protecting your child.
Excuse me, but that is not protecting the child, that is protecting the FATHER from his child knowing what a douche he really is. That's like a mother telling her children that "Daddy went on a business trip for a few years", when really his ass is behind bars. Why shouldn't children know exactly what kind of people there parents are?