Call in 作"请来"、"找来"、"召来"解,"请人来作某种专业的咨询或帮忙"的意思。如:
Your father is ill, you should call in a doctor at once.你父亲病了,应该马上去请个大夫来。
The police have been called in to help make it clear.
已请来了警察帮忙把这件事弄清楚。

Call on和call at的意思一样,都是"拜访""访问",但call on后接表示人的名词或代词,call at 后接表示地点或场所的词语。试比较:
I called on the Smiths yesterday.
我昨天拜访了史密斯一家人。
I called at the Smith's yesterday.
我昨天去了史密斯家。
I called on the doctor yesterday.
我昨天去拜访了医生。
I called at the doctor's yesterday.
我昨天去了医务室。
另外,call on还可以"号召",后面往往接动词不定式作宾语补主语。如:
The headmaster called on the students to work harder.校长号召学生们更努力地学习。
 

Call for 有几个不同的意思:
1.作"要求"、"需要"、"提倡"解。如:
Success calls for hard work.
成功要靠勤奋。
People are calling for the freedom of all slaves.人们要求解放所有奴隶。
The work calls for a lot of time.
这项工作需要大量时间。
2.表示"接(某人)"、"来取(某物)"。如:
I'll call for you at sever this evening.
今晚七点我来接你。
We call for the package at the post office.我们到邮局去取包裹。
3.表示"喊着要人取来",相当于ask for。如:
The man sat down and called for a glass of beer.
那人坐下来,要了一杯啤酒。

Call up是"打电话"之意,相当于ring up或telephone或phone。 如:
I'll call you up tomorrow.
我明天给你打电话。
另外,call up 还有"回想起"之意。如:
Your letter called up the days when we worked together.你的信使我回想起我们一同工作的日子