1. formally ask somebody or require a particular attribute to do something
call upon someone to speak
To call on someone or something or to call someone to do something means to officially ask a person or organisation to act in some way. To call on someone can also mean to invite or ask someone to speak. The phrasal verb to call upon has a similar meaning which is to ask or demand that somebody do something or to invite someone to make a short speech.
2. visit somebody for a short time
call on somebody to see how they are
To call on someone can mean to visit somebody for a short time usually for a social visit or to check that they are okay. This use of the phrasal verb is more often used in a social context.