Tôi gọi dịch vụ khách hàng để khóa thẻ.

A word-by-word breakdown is:

• Tôi = I
• gọi = call, phone
• dịch vụ khách hàng = customer service
• để = in order to, to
• khóa thẻ = lock/block the card

So the structure is basically:

Tôi + gọi + dịch vụ khách hàng + để + khóa thẻ
= I call customer service to block my card

Để introduces a purpose.

In this sentence, it connects:

• gọi dịch vụ khách hàng = the action
• khóa thẻ = the purpose of that action

So để khóa thẻ means:

• to block the card
• in order to block the card

This is very common in Vietnamese.

Examples:

• Tôi học tiếng Việt để nói chuyện với bạn bè.
  = I study Vietnamese to talk with friends.
• Cô ấy đến ngân hàng để mở tài khoản.
  = She went to the bank to open an account.

Yes. Gọi can mean to call, and in many contexts it naturally means to make a phone call.

So Tôi gọi dịch vụ khách hàng means I call customer service.

Depending on context, gọi can also mean:

• to call someone’s name
• to summon
• to order something in a restaurant

But here, with dịch vụ khách hàng, the meaning is clearly phone call.

You could also hear:

• Tôi gọi điện cho dịch vụ khách hàng
  This is more explicit: I phone customer service.

It is understandable, but many speakers would more naturally say:

• chăm sóc khách hàng = customer care/customer service
• bộ phận chăm sóc khách hàng = customer service department

So in everyday Vietnamese, these may sound more natural:

• Tôi gọi chăm sóc khách hàng để khóa thẻ.
• Tôi gọi bộ phận chăm sóc khách hàng để khóa thẻ.

That said, dịch vụ khách hàng is still understandable and not wrong in many contexts. It just may sound a little less idiomatic than chăm sóc khách hàng.

Vietnamese often leaves out information that is already obvious from context.

If you are calling customer service to block a card, listeners will usually assume it is your card unless you say otherwise. So:

• khóa thẻ = block the card
• khóa thẻ của tôi = block my card

Both are possible. The version without của tôi is shorter and very natural.

If you need to be extra clear, you can say:

• Tôi gọi chăm sóc khách hàng để khóa thẻ của tôi.

Literally, yes: khóa means lock and thẻ means card.

But in banking context, khóa thẻ usually means:

• freeze the card
• block the card
• disable the card temporarily or immediately

This is the natural Vietnamese expression for stopping the card from being used.

A related expression is:

• hủy thẻ = cancel the card

So:

• khóa thẻ = block/freeze it
• hủy thẻ = cancel it completely

By itself, thẻ just means card. In context, it often means:

• bank card
• debit card
• credit card
• ATM card

If you want to be more specific, Vietnamese can say:

• thẻ ngân hàng = bank card
• thẻ tín dụng = credit card
• thẻ ghi nợ = debit card
• thẻ ATM = ATM card

In your sentence, thẻ works because the situation makes the meaning clear.

Vietnamese does not require tense marking the way English does.

So Tôi gọi dịch vụ khách hàng để khóa thẻ could mean different things depending on context:

• I call customer service to block my card
• I’m calling customer service to block my card
• I called customer service to block my card

If you want to make the time clearer, you add time words or aspect markers:

• Tôi đã gọi dịch vụ khách hàng để khóa thẻ.
  = I called customer service to block the card.
• Tôi đang gọi dịch vụ khách hàng để khóa thẻ.
  = I am calling customer service to block the card.
• Ngày mai tôi sẽ gọi dịch vụ khách hàng để khóa thẻ.
  = Tomorrow I will call customer service to block the card.

Sometimes, yes, but it depends on context.

In Vietnamese, pronouns are often dropped when the subject is obvious. So in conversation, someone might simply say:

• Gọi dịch vụ khách hàng để khóa thẻ.

However, this can also sound more like:

• a note
• an instruction
• a general statement

If you want a clear full sentence meaning I call / I called..., keeping tôi is safest.

In Vietnamese, the main noun usually comes first, and the word that modifies it comes after.

So:

• dịch vụ khách hàng = service for customers / customer service

Here:

• dịch vụ = service
• khách hàng = customer

The second part narrows or explains the first. This word order is very common in Vietnamese noun phrases.

For example:

• ngân hàng nhà nước = state bank
• trung tâm thương mại = shopping center

So dịch vụ khách hàng follows a normal Vietnamese pattern.

Yes, the sentence is understandable, but a more natural everyday version would often be:

• Tôi gọi chăm sóc khách hàng để khóa thẻ.
• Tôi gọi lên ngân hàng để khóa thẻ.
• Tôi gọi cho ngân hàng để khóa thẻ.

These alternatives may sound more conversational because people often say call the bank or call customer care rather than call customer service in a more literal way.

So your original sentence works, but if your goal is very natural spoken Vietnamese, chăm sóc khách hàng is a strong option.

A rough pronunciation guide is:

• Tôi ≈ toy
• gọi ≈ somewhat like goy with a falling/glottalized tone
• dịch vụ ≈ zik voo or yik voo depending on accent
• khách hàng ≈ khak hang
• để ≈ day with a low-falling tone
• khóa ≈ khwah
• thẻ ≈ teh with a dipping/glottalized tone

A few tricky points:

• kh is like a breathy k sound, somewhat similar to the ch in German Bach for some learners.
• th in Vietnamese is not the English th sound. It is an aspirated t, closer to a strong t.
• Tones matter a lot, especially in words like gọi, để, khóa, and thẻ.

If you want to sound natural, it helps to practice the sentence in chunks:

• Tôi gọi
• dịch vụ khách hàng
• để khóa thẻ

Usually, để is the normal choice here.

It is the standard way to express purpose:

• Tôi gọi ... để khóa thẻ.

In some contexts, Vietnamese also uses other structures for purpose, but for a learner, để + verb is the most useful and natural pattern to remember.

So for this sentence, để is exactly what you want.