keitaidenwagaisya ni denwasuru.

It is pronounced けいたいでんわがいしゃ.

It breaks down like this:

• 携帯電話（けいたいでんわ） = mobile phone / cell phone
• 会社（かいしゃ） = company

So 携帯電話会社 literally means mobile-phone company, and in natural English it usually means a cell phone carrier or mobile service provider.

With 電話する, the particle に marks the person or organization you are calling.

So:

• 携帯電話会社に電話する = call the mobile phone company

This is different from English, where we just say call the company with no extra word. In Japanese, に shows the target of the call.

More examples:

• 友だちに電話する = call a friend
• 病院に電話する = call the hospital

Yes. 電話 is basically a noun meaning telephone or phone call.

Japanese often makes verbs by combining a noun with する:

• 勉強する = to study
• 運動する = to exercise
• 電話する = to call / to phone

So 電話する is a very common noun + する verb.

Yes. Both are possible:

• 電話する
• 電話をする

In everyday Japanese, 電話する is very common and natural.

If you include を, it marks 電話 as the thing being done:

• 携帯電話会社に電話をする

That is also correct, but often the を is omitted in this kind of noun + する verb.

No, that is not natural.

With 電話する, the company or person being called is normally marked by に, not を.

Correct:

• 携帯電話会社に電話する

Not natural:

• 携帯電話会社を電話する

If you want to use を, it usually attaches to 電話 instead:

• 携帯電話会社に電話をする

Yes. This is very normal in Japanese.

Japanese often leaves out the subject when it is understood from context. So this sentence could mean things like:

• I will call the mobile phone company.
• (You should) call the mobile phone company.
• (Let’s) call the mobile phone company.

The exact subject depends on the situation.

電話する is the plain, dictionary-form, non-past form.

Non-past in Japanese can cover several meanings depending on context, such as:

• call / calls
• will call
• to call

So by itself, 携帯電話会社に電話する。 could mean:

• I call the mobile phone company
• I will call the mobile phone company

Context decides which one is intended.

Related forms:

• 電話します = polite non-past
• 電話した = past
• 電話しない = negative
• 電話してください = please call

It is grammatically correct and easy to understand, but in everyday conversation people might often use a shorter or more natural expression depending on context.

For example:

• 携帯会社 = cell phone company
• 通信会社 = telecom company
• キャリア = carrier
• the actual company name, such as ドコモ or ソフトバンク

So 携帯電話会社に電話する is correct, but in real life a speaker might say something shorter.

No. Standard Japanese is normally written without spaces.

So this would usually be written as:

携帯電話会社に電話する。

Spaces are often added in textbooks or learning materials just to make the structure easier to see.

Normally, no. With 電話する, に is the standard particle.

• 携帯電話会社に電話する = natural
• 携帯電話会社へ電話する = not the usual choice

The particle へ is mainly used for direction or movement, and a phone call target is usually marked with に.

Yes. A very common alternative is 電話をかける.

So you can say:

• 携帯電話会社に電話する
• 携帯電話会社に電話をかける

Both mean to call the mobile phone company.

A rough difference:

• 電話する = very common, simple to call
• 電話をかける = also very common, slightly more explicitly to make/place a call

Both are natural.