To call up means to telephone someone. When you call up your grandmother to tell her about your day, you're picking up the phone and dialing her number. Before cell phones existed, people would say “I'll call you up later” to mean they'd telephone when they got home.

The phrase can also mean to summon someone to active duty or to bring someone onto a team. When a baseball player gets called up from the minor leagues to the majors, the team is promoting them to the highest level of professional play. During wartime, countries might call up military reserves, ordering trained soldiers to report for duty.

You might also call up a memory or feeling, bringing it to mind. A familiar smell can call up memories of your childhood home. Teachers sometimes call up students to solve problems at the board, asking them to come forward from their seats.

The meaning usually depends on context. If someone says “call me up,” they want you to phone them. But if a coach says “I'm calling you up,” you might be joining the varsity team.