Call in 作"請來"、"找來"、"召來"解,"請人來作某種專業(yè)的咨詢或幫忙"的意思.如：
Your father is ill,you should call in a doctor at once.你父親病了,應該馬上去請個大夫來.
The police have been called in to help make it clear.
已請來了警察幫忙把這件事弄清楚.
Call on和call at的意思一樣,都是"拜訪""訪問",但call on后接表示人的名詞或代詞,call at 后接表示地點或場所的詞語.試比較：
I called on the Smiths yesterday.
我昨天拜訪了史密斯一家人.
I called at the Smith's yesterday.
我昨天去了史密斯家.
I called on the doctor yesterday.
我昨天去拜訪了醫(yī)生.
I called at the doctor's yesterday.
我昨天去了醫(yī)務室.
另外,call on還可以"號召",后面往往接動詞不定式作賓語補主語.如：
The headmaster called on the students to work harder.校長號召學生們更努力地學習.