call on / upon:
1. call on sb. to do sth.- you say in public that you want them to do that.
e.g. We should call on all our resources to rescue wild creatures which are in the danger of extinction.
<c.f> She called on / appealled to / asked for us to help her villagers. 她請求我們去幫助她的村民.
2. call on sb.- you pay him a short visit.
e.g. He intended to call on his grandma. 他想去看他的祖母.
1. call for sb.: you go to the building where he is, so that you can see he goes somewhere.
e.g. I'll call for you / pick you up at 11 a.m.
2. call for sth.: you demand that it should happen.
e.g. Claire blisteringly called for her boyfriend carrying on her the back by foot for a mile because of his late arrival.
3. sth. calls for a particular action or quality: it needs or makes it necessary.
e.g. The situation calls for prompt / immediate action.
Collins Cobuild Advanced Learner's English Dictionary New Edition, 文馨當代英漢字典第五版, mich