naisen de butyou ni denwasimasu.

It is read:

ないせん で ぶちょう に でんわします。

In normal Japanese writing, you would usually see it without spaces:

内線で部長に電話します。

The spaces here are just to help learners see the parts of the sentence.

内線 means an internal phone line or extension inside a company, school, hotel, and so on.

So it does not mean a regular outside phone call. It specifically refers to calling through the organization’s internal phone system.

Here, で shows the means or method used to do something.

So 内線で means:

• by extension
• using the internal line
• on the internal phone system

This is a very common use of で in Japanese: it marks the tool, method, or medium.

Examples:

• バスで行きます = I go by bus
• 日本語で話します = I speak in Japanese
• 内線で電話します = I call by extension

に marks the target of the phone call.

In this sentence, 部長に電話します means call the department manager.

With 電話する, the person you call is very often marked with に.

So:

• 母に電話します = I call my mother
• 先生に電話します = I call the teacher
• 部長に電話します = I call the department manager

電話する is a very common noun + する verb pattern. In everyday Japanese, the を is often omitted.

So these are both possible:

• 電話します
• 電話をします

But 電話します is very natural and common.

You may also hear:

• 電話をかけます

That also means to make a phone call.

So:

• 部長に電話します
• 部長に電話をします
• 部長に電話をかけます

All are possible, though the first and third are especially common.

します is the polite non-past form.

In Japanese, the non-past can mean:

• a present/habitual action, or
• a future action

So depending on context, this could mean:

• I call the manager by extension
• I will call the manager by extension

Japanese often leaves the exact time to context unless a time word is added.

The subject is omitted, which is very normal in Japanese.

A natural English translation might include:

• I
• we
• sometimes another understood subject

So the full idea might be:

• I’ll call the department manager on the extension line

Japanese often leaves out words that are already understood from context.

In Japanese, job titles are often used like names.

So 部長 can mean:

• the department manager
• Manager
• the boss in that role

It is very common to address or refer to someone by title in workplace Japanese.

Examples:

• 社長 = company president
• 課長 = section manager
• 部長 = department manager

So 部長に電話します sounds very natural in a company setting.

Yes, to some extent. Because Japanese uses particles like で and に, the roles of the words are clear even if the order changes.

For example, these are both understandable:

• 内線で部長に電話します
• 部長に内線で電話します

Both mean basically the same thing.

However, the original order is very natural:

• method/tool first: 内線で
• target next: 部長に
• verb last: 電話します

Japanese verbs normally come at the end of the sentence.

Not in this sentence if you mean call someone.

With 電話する, the person being called is usually marked with に:

• 部長に電話します

If you use と, it usually suggests with someone, not to someone. That changes the meaning and sounds unnatural here.

If you want to say talk with the manager on the phone, you would use a different structure, such as:

• 部長と電話で話します = I talk with the manager on the phone

So:

• に = the person you call
• と = the person you talk with

Yes. 電話します uses the polite ます form, so the sentence is polite in style.

A plain-form version would be:

• 内線で部長に電話する。

The polite form is appropriate in many workplace or classroom situations, which fits this sentence well.