这两个短语都表示“拜访”，但是用法有所差别。

1.drop in on

解析：drop in on后面接表示人的名词或代词。

I thought I'd drop in on you while I was passing.

我曾想路过时顺便来看看你。

It's improper to drop in on him so casually.

这样冒冒失失去找他可不好。

I dropped in on him on my way home.

我在回来的路上趁便去他家看了看。

One day, the poor man dropped in on a wise old man and asked him for advice about changing his life.

一天,这个可怜的人拜访了一位聪明的老人,向他请教如何改变他的生活。

They dropped in on us as they were passing through town.

他们经过镇子的时候顺便拜访过我们。

On my way to the park, I dropped in on a friend of mine.

在我去公园的路上，我顺道看望了我的一个朋友。

He dropped in on some old comrades on his vacation trip to Beijing.

他在去北京度假时顺便访问一些老同志。

2.drop in at

解析：drop in at后面接表示处所的名词或代词。

On my way home I dropped in at his house.

在我回家的路上，我顺便到他家一趟。

He dropped in at our school yesterday.

他昨天顺便拜访了我们学校。

Let’s drop in at Tom’s.

让我们到汤姆家坐坐。

I dropped in at the club on my way home.

在回家的路上我顺便去了俱乐部。

Please drop in at my office when you happen to be free.

如果你碰巧有空的话，请到我的办公室来。

The Secretary asked me to drop in at his office.

书记让我到他办公室去找他。

Shall we drop in at a coffee shop?

我们顺便去一下咖啡店好吗？

Won't you drop in at our house and have tea?

你不想顺便来一下我们家喝杯茶吗？

I will call on him tomorrow.

我明天将去拜访他.

I'll call on you in my spare time.

有空我去找你。

He just calls in occasionally...

他只是偶尔过来坐坐。

Builders were called in to make the building safe.

召来建筑工加固这栋大楼。

We had to call in a plumber to unblock the drain.

我们只得叫个管子工来疏通下水道。

Public health officials were called in to inspect the premises.

公共卫生官员奉召来视察了建筑物。

The military was/ were called in to deal with the riot.

已调来军队平息暴乱。

Troops were called in to subdue the rebels.

军队被调来镇压反叛者。

Forensic experts were called in

法医专家被请来。

Call in an architect or surveyor to oversee the work.

找个建筑师或监理员来监工。

The FBI has been called in to investigate.

联邦调查局奉命进行调查。

Emma has just called in sick.

埃玛刚才打电话来请病假了。

I called in at my local on the way home.

我回家途中去了我住处附近的酒吧。《牛津高阶英汉双解词典》

I'll call in at the chemist's on my way home.

我回家时顺便要去药店一趟。