Call up and Sign up

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

Sign up and call up are semantically related in gather topic. In some cases you can use "Sign up" instead a verb "Call up", when it comes to topics like gather resources, select for military force.

Sign up verb - Engage by written agreement.

Call up and sign up are semantically related in gather topic. You can use "Call up" instead a verb "Sign up", if it concerns topics such as gather resources, select for military force.