Call up and Get on line

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

Get on line and call up are semantically related In some cases you can replace phrase "Get on line" with "Call up", this terms are similar.

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