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.