call at 地点 ：拜访某地; 拜访，访问；停靠（车站）
call in at 地点 ：拜访某地
call on 人： 拜访某人; 访问，拜访；号召，请求
call in 请来，找来；打电话进来
call off 取消
call in on 人 拜访某人（一般时到某人的家中拜访）
经典例句：
I'll now call at the vicarage and report to you in due course.
我现在要去教区牧师家拜访，然后适时向你汇报情况。
A market researcher called at the house where he was living
一位市场调查人员造访了他的住处。
The steamer calls at several ports along the way.
一路上，汽船在好几个港口停靠。
I'm going to call on one of my former classmates.
我要去看望我的一位老同学。（拜访）
You'd better call on your friends to help you.
你最好请朋友来帮你。（请求）
That's what the Party called on us to do.
那是党号召我们做的事。（号召）
A mother buyer wanted Chen Xiao to call in on her young daughter disguised as a panda, and encourage her to study harder, after becoming concerned by her failure of an exam.
When she went to the shops, she used to call in on her mother to see if she wanted anything.
一位母亲买者，在受到女儿考试失利的困扰之后，想让陈潇扮成熊猫，前来拜访她女儿并鼓励她努力学习。（经典例句）
I got into the habit of calling in on Gloria on my way home.
我养成了回家路上顺便去看格洛丽亚的习惯。
《柯林斯高阶英汉双解学习词典》
她在去商店买东西的时候常常去她母亲那，看母亲是否要买些东西。
Could you call in on Mum on your way home?
你能在回家的路上顺便看看妈妈吗？
Please call in on me any time you're in town.
你什么时候来城里就顺便来坐坐。
Let's call in on the old granny for some minutes.
我们花几分钟去看看老奶奶吧。
I formed the habit of calling in on them in the evenings.
我每天晚上都到他们那里去，这已成了习惯。
call in at 与 call at 的区别？——这个两个短语也是“顺便拜访，去一趟”，后接“地点”，不是“某人”——这是跟call in on 不同的地方。
还有哪些动词后面接 on 然后跟 sb？——这个有很多：look down on sb.(看不起), rely on sb., （依靠）depend on sb.（依靠）等等。
Call in an architect or surveyor to oversee the work.
找个建筑师或监理员来监工。
《柯林斯高阶英汉双解学习词典》
People call in and just spill their guts about whatever's bothering them on the job or in a relationship.
人们打进电话，就是要将工作或感情上的所有困扰倾诉出来。
《柯林斯高阶英汉双解学习词典》
He has called off the trip.
他已取消了这次行程。
Greenpeace refused to call off the event.
绿色和平组织拒绝取消这一活动。
Since the weather is bad, let's call off the trip.
既是天气不好，那就不去了吧。
Romania's government issued a last-minute appeal to him to call off his trip.
罗马尼亚政府最后一刻恳请他取消行程。
The union threatened a strike but called it off at the last minute.
工会威胁要罢工，不过在最后一刻取消了行动。
《柯林斯高阶英汉双解学习词典》
