Насчёт встречи я позвоню тебе вечером.

Насчёт means about, regarding, or concerning.

So Насчёт встречи means about the meeting.

In this sentence, it introduces the topic of the phone call:

• Насчёт встречи = about the meeting
• я позвоню тебе вечером = I’ll call you in the evening

A very similar alternative is о встрече, but насчёт is especially common in everyday speech.

Because насчёт requires the genitive case.

The noun встреча is nominative singular. Its genitive singular form is встречи.

So:

• встреча = meeting
• насчёт встречи = about the meeting

This is something you simply have to learn with the preposition:
насчёт + genitive

They are similar, but not always identical in tone.

• насчёт + genitive = about / regarding
• о + prepositional = about

Examples:

• Насчёт встречи
• О встрече

Both can often be translated as about the meeting, but насчёт often sounds a bit more practical or conversational, especially in phrases like:

• Насчёт работы
• Насчёт завтра
• Насчёт встречи

So in this sentence, насчёт feels very natural and idiomatic.

Позвоню is the future tense of the perfective verb позвонить.

Russian often uses a perfective future when talking about a single completed action in the future:

• я позвоню = I will call

Compare:

• звоню = I am calling / I call (present tense, imperfective)
• буду звонить = I will be calling / I will call (imperfective future, often with focus on process or repeated action)
• позвоню = I will call (one completed call)

Here the speaker means one future call, so позвоню is the most natural choice.

Because the verb позвонить takes the dative case for the person you call.

So:

• позвонить кому? = to call whom?
• тебе = to you (dative)

Examples:

• Я позвоню тебе. = I’ll call you.
• Она позвонила мне. = She called me.
• Мы позвоним маме. = We’ll call Mom.

English says call someone, but Russian uses a structure closer to call to someone.

Because Russian often uses the instrumental case to express time when something happens, especially with parts of the day.

So:

• утром = in the morning
• днём = in the daytime / in the afternoon
• вечером = in the evening
• ночью = at night

That is why Russian says:

• я позвоню вечером = I’ll call in the evening

No preposition is needed here.

Yes, я could be omitted.

Russian verb endings usually already show the subject:

• позвоню already means I will call

So both are possible:

• Насчёт встречи я позвоню тебе вечером.
• Насчёт встречи позвоню тебе вечером.

Including я can add a little emphasis or clarity, especially if the speaker wants to stress I rather than someone else.

Yes, the word order can be changed.

Russian word order is more flexible than English. The sentence starts with Насчёт встречи because that is the topic being introduced first:

• As for the meeting, I’ll call you this evening.

Other possible orders:

• Я позвоню тебе вечером насчёт встречи.
• Вечером я позвоню тебе насчёт встречи.
• Я вечером позвоню тебе насчёт встречи.

These all mean roughly the same thing, but the emphasis changes slightly.

In your sentence, putting Насчёт встречи first makes the topic very clear from the start.

Yes, here it is one word: насчёт.

This is the preposition meaning about / regarding:

• насчёт встречи = about the meeting

But на счёт as two words is a different expression, usually meaning something like onto an account or to the account:

• положить деньги на счёт = to put money into an account

So in your sentence, only насчёт is correct.

Literally, вечером means in the evening.

But depending on context, English may naturally translate it as:

• in the evening
• this evening
• sometimes even tonight

For example:

• Я позвоню тебе вечером can be translated as I’ll call you this evening or I’ll call you tonight, depending on the situation.

So the Russian word is a bit broader than one single fixed English phrase.

Yes, it sounds very natural.

Насчёт встречи я позвоню тебе вечером is a normal, conversational sentence. A native speaker would easily say something like this when they want to discuss the meeting later.

Other natural variants include:

• Я позвоню тебе вечером насчёт встречи.
• Насчёт встречи позвоню тебе вечером.

All of these sound normal; the difference is mainly emphasis and information structure.