keiyakusya ga keitaidenwagaisya ni denwasuru.

A common reading is:

けいやくしゃ が けいたいでんわがいしゃ に でんわする。

Word by word:

• 契約者 = けいやくしゃ
• が = が
• 携帯電話会社 = けいたいでんわがいしゃ
• に = に
• 電話する = でんわする

Japanese is normally written without spaces, so the spaced version is just for learners.

契約者 literally means the person who has the contract.

In this kind of sentence, it often means:

• subscriber
• contract holder
• account holder
• sometimes customer, depending on context

So it is a little more specific than a general word like customer. It refers to the person whose name is on the phone contract.

が marks the subject of the sentence.

Here, 契約者が means that the contract holder/subscriber is the one performing the action.

So:

• 契約者が = the subscriber is the one who...
• 電話する = calls / will call

In a basic explanation, が tells you who does the action.

With 電話する, the person or organization you call is commonly marked with に.

So:

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

This may feel unusual to English speakers, because in English we say call someone directly, without a preposition. But in Japanese, に is very natural here.

You can think of it as:

• 電話する = make a phone call
• 会社に = to the company

This is a very common learner question.

Even though English says call the company, Japanese usually says:

• 会社に電話する

not

• 会社を電話する

That is because 電話する does not usually take the person called as a direct object with を.

Instead, the target of the call is marked with に.

So the pattern is:

• X に 電話する = call X

電話する is a suru-verb.

That means it is made from the noun 電話 (phone call / telephone) plus する (to do), giving:

• 電話する = to call / to make a phone call

This is a very common pattern in Japanese:

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

It can mean either, depending on context.

Japanese non-past forms often cover both:

• present / habitual: calls
• future: will call

So this sentence could mean:

• The subscriber calls the mobile phone company.
• The subscriber will call the mobile phone company.

Context tells you which one is intended.

The basic order here is:

• subject + target + verb

So:

• 契約者 が 携帯電話会社 に 電話する

Because Japanese uses particles like が and に, word order is more flexible than in English. For example, you may also see:

• 携帯電話会社に契約者が電話する

However, the most neutral and beginner-friendly order is the original one.

Also, the verb usually comes at the end.

Yes, it is a compound noun. It breaks down like this:

• 携帯電話 = mobile phone / cell phone
• 会社 = company

So:

• 携帯電話会社 = mobile phone company / cell phone company / carrier

It is a very normal Japanese-style compound noun, where several nouns are put together into one larger noun.

They are only there to help learners.

Normal Japanese would usually be written like this:

契約者が携帯電話会社に電話する。

Japanese does not normally separate words with spaces the way English does.

A polite version would be:

契約者が携帯電話会社に電話します。

Here, する becomes します.

So:

• 電話する = plain form
• 電話します = polite form

If you were speaking in conversation with a customer or in a business setting, 電話します would usually be more natural.

Yes, you sometimes can, but に is more standard and common with 電話する.

• 会社に電話する = the usual phrasing
• 会社へ電話する = also possible, with a slight sense of direction toward the company

For most learners, it is best to learn:

• 人／会社 に 電話する

Not exactly.

• 契約者 = the contract holder / subscriber
• お客さん = customer

お客さん is broader and more general.
契約者 is more specific: it emphasizes the person who officially signed or holds the contract.

So if a phone company is talking about the person responsible for the account, 契約者 is often the better word.