请采纳为答案
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.
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.