call on “拜访；号召,请求”
call in “召集；召来”
call on 比较常用来表达“拜访”,call in 更多时候是“号召,召集”的意思
例1.I will call on him tomorrow.
我明天将去拜访他.
例2.Call in the troops and clear the bazaars.
召集军队,荡平集市区!
call in 偶尔也可以表示“拜访”的意思,但是它月call on 的区别就在于call on是及物动词词组,也就是call on后面可以直接加人物名称,相反地,call in 后面必须先加介词,然后才能加人物.也就是call in on…或者call in to do …
例3.Oh,Paul,what a pity you didn't call in to see Mrs Brown.
噢,保罗,你没有顺便拜访一下布朗夫人,真遗憾!