be on the horn

1. To make use of or speak (to someone) on the telephone. Don't disturb me, kids, I'm on the horn to your father at the moment. After our dreadful meal that night, I was on the horn to the manager the very next morning.

2. dated To be lustful or sexually excited, especially of a man; to have an erection. Sarah became quite embarrassed when she noticed that her date was on the horn in the middle of the movie.