Call up and Get on the horn

Call up verb - Get or try to get into communication (with someone) by telephone.

Get on the horn and call up are semantically related in telephone topic. In some cases you can use "Get on the horn" instead a verb "Call up", when it comes to topics like communicate through telephone system.

Call up and get on the horn are semantically related in telephone topic. You can use "Call up" instead a verb phrase "Get on the horn", if it concerns topics such as communicate through telephone system.