call on与call at都表示"拜访",两者区别在于后面所接宾语的不同.call on后面接somebody,而call at后面接some place.
a. I called on him yesterday.我昨天拜访了他.
b.I called at his house yesterday.我昨天拜访了他家.
此外,call on/upon还可以表示"号召",构成词组call on/upon somebody to do something "号召某人做某事".
We are called on to make some personal decisions every day.每天我们被号召去做一些个人决定.