The phrase "got someone on horn" is a colloquialism that refers to making a phone call. If you're looking for synonyms to use in place of this expression, there are several options. You could say that you're giving someone a ring, dialing them up, phoning them, calling them, or reaching out to them. Other alternatives include saying that you're placing a call, making a connection, getting in touch, or speaking with someone over the phone. No matter which synonym you choose, the important thing is that you're communicating with someone using the telephone.