Learn Hardcore Vietnamese: Đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng. - Don't call me on the phone when I am in a meeting at the office.

Đừng gọi điện thoại cho tôi…
Very clear: Don’t call me on the phone…
Đừng gọi cho tôi…
Also very common in speech. Context usually makes it clear this means don’t phone me, not don’t call out to me.
Đừng điện thoại cho tôi…
Also possible, especially in the South, meaning don’t phone me.

So you don’t have to use both words, but gọi điện thoại is very explicit and neutral, and what you have in the sentence is perfectly natural.

What does cho tôi do in gọi điện thoại cho tôi? Is it necessary?

Cho often marks the indirect object (the person who receives something or is affected by the action).

gọi điện thoại = to make a phone call
gọi điện thoại cho tôi = to call to me / for me

So cho tôi answers “to whom?” → to me.

Is it necessary?

Without it: Đừng gọi điện thoại khi tôi đang họp…
This is grammatical, but sounds more general: Don’t make phone calls when I’m in a meeting…
With cho tôi: Đừng gọi điện thoại cho tôi khi…
Very clear: Don’t call *me when…*

So if you specifically mean “me”, cho tôi is natural and recommended.

What does đang mean in tôi đang họp?

Đang marks an ongoing action (similar to English “am/is/are -ing”):

tôi họp = I (have) a meeting / I meet (neutral, no aspect)
tôi đang họp = I am in the middle of a meeting; I’m currently meeting

So khi tôi đang họp ở văn phòng focuses on the time while the meeting is in progress.

You can say:

Đừng gọi điện thoại cho tôi khi tôi đang họp.
Don’t call me while I am in a meeting.

Leaving đang out is still okay, but less clear that the action is in progress at that exact time.

Could we say khi tôi họp instead of khi tôi đang họp? What is the difference?

Yes, khi tôi họp is grammatically correct, but the nuance changes slightly:

khi tôi đang họp
Emphasizes the ongoing process → when I’m (in the middle of) having a meeting.
khi tôi họp
More neutral → when I have a meeting (can sound a bit more general or habitual).

In real use, people often choose đang here because we usually mean “don’t call me while I’m actively in a meeting”, not “on days when I have meetings, in general”.

Can we omit the second tôi and just say khi đang họp ở văn phòng?

Yes, that is possible:

Đừng gọi điện thoại cho tôi khi đang họp ở văn phòng.

In Vietnamese, the subject is often dropped if it is clear from context. Here, we already know we are talking about tôi, so it is understandable.

However:

khi tôi đang họp ở văn phòng is:
- slightly clearer (especially in writing or more formal speech),
- a bit more complete sounding.

Both are natural. In careful or formal Vietnamese, repeating tôi is quite normal.

Can we move khi tôi đang họp ở văn phòng to the beginning of the sentence?

Yes. Both orders are correct:

Đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng.
Khi tôi đang họp ở văn phòng, đừng gọi điện thoại cho tôi.

The meaning is the same. Vietnamese is flexible about placing time clauses:

Time clause at the beginning often sets the context first.
Time clause at the end is also very common in speech.

Just remember to keep the comma in writing in option 2 to show the pause.

What is the difference between ở văn phòng and tại văn phòng?

Both can mean “at the office / in the office”, but they differ in tone:

ở văn phòng
- Very common in everyday speech.
- Neutral and natural in most contexts.
tại văn phòng
- More formal / written.
- Common in documents, announcements, official writing.

In your sentence, ở văn phòng is perfectly natural conversational Vietnamese:

Đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng.

Is this sentence polite, or could it sound rude? How can I soften it?

Đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng. is:

Direct and clear.
Neutral in politeness by grammar, but in English terms it can feel quite blunt depending on tone and relationship.

To soften it, you can add polite elements:

Làm ơn đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng.
= Please don’t call me when I’m in a meeting at the office.
Anh/Chị đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng nhé.
- Using anh / chị to address the listener respectfully.
- Adding nhé softens the command.

Politeness in Vietnamese is strongly affected by:

Which pronouns you use (anh, chị, em, cô, chú, con, cháu…),
Small particles like ạ, nhé, nha, với,
Your tone of voice.

How does this sentence express future time without a word like “will” or “won’t”?

Vietnamese does not mark tense the same way English does. There is no mandatory “will” word.

The sentence:

Đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng.

is understood as future-oriented because:

Đừng is a negative command → talking about what should not happen from now on.
The context “when I’m in a meeting” is obviously about times that will occur in the future.

If you really wanted to stress “in the future”, you could add time phrases:

Từ giờ trở đi, đừng gọi điện thoại cho tôi khi tôi đang họp ở văn phòng.
= From now on, don’t call me when I’m in a meeting at the office.

What is họp exactly? Is it a verb or a noun?

Họp in this sentence is a verb: to meet (in a meeting), to hold a meeting.

Tôi đang họp. = I am in a meeting / I’m having a meeting.

Related forms:

cuộc họp = a meeting (noun phrase)
- Tôi có một cuộc họp. = I have a meeting.
phòng họp = meeting room.

So:

tôi đang họp → I am currently in a meeting (verb).
tôi có cuộc họp → I have a meeting (noun phrase).

Why is it gọi điện thoại cho tôi and not gọi điện thoại tôi?

In Vietnamese, the usual pattern for “call someone (on the phone)” is:

gọi (điện thoại) cho + [person]

The cho marks the person who receives the call.

gọi điện thoại cho tôi = call me (on the phone)
gọi cho mẹ = call mom
gọi điện cho anh ấy = call him (on the phone)

Using gọi điện thoại tôi (without cho) is not natural Vietnamese for “call me”. You really need cho here to make it idiomatic.