How can I understand "up" here? What's different between "call me" and "call me up"?
Native speaker’s answer
Rebecca
To "call someone up" means to call them by phone. It has the same meaning as "call me" but is used much more casually and informally. Ex: Why don't you call me on Friday? Ex: Why don't you call me up on Friday? Ex: I'll call you up when I know the answer. Ex: I'll call you when I know the answer.