Мне некому позвонить вечером.

Because this is an impersonal construction. Russian often uses the dative to show the person affected by the situation.

So мне некому позвонить is literally something like:

To me, there is no one to call.

In more natural English, that becomes I have no one to call.

Here, мне does not mean to me in the usual direct sense of the verb позвонить. It marks the experiencer: the person whose situation this is.

Некому means there is no one to... or nobody to..., in a special impersonal pattern.

It comes from не + кому, but in practice you should learn it as a fixed form used in sentences like:

• Мне некому помочь. — I have no one to help me.
• Нам не с кем поговорить. — We have no one to talk to.
• Ей некуда идти. — She has nowhere to go.

So in Мне некому позвонить вечером, некому means there is no one for me to call.

This is a very common question.

• Никому is the dative form of никто and is normally used with a finite verb:

  • Я никому не звоню. — I call nobody / I’m not calling anybody.

• Некому is used in an impersonal infinitive construction:

  • Мне некому позвонить. — I have no one to call.

So the difference is not just vocabulary; it is also about the sentence pattern.

Compare:

• Я никому не звоню. — I am calling nobody.
• Мне некому позвонить. — There is nobody available for me to call.

Because the construction мне некому + infinitive means I have no one to do X with / no one to X.

There is no ordinary subject-verb structure here. Instead, Russian uses an impersonal pattern:

• мне — for me
• некому — there is no one
• позвонить — to call

So the infinitive expresses the action that cannot be carried out because no suitable person exists.

This same pattern appears in many sentences:

• Мне некого спросить. — I have no one to ask.
• Им негде жить. — They have nowhere to live.
• Нам незачем ждать. — We have no reason to wait.

Because позвонить is perfective, and here it usually means to make a call / to call once.

That is the most natural idea in this sentence: I have no one to call this evening / in the evening.

If you used звонить, the meaning would feel more like an ongoing or repeated activity:

• Мне некому позвонить вечером. — I have no one to call this evening / in the evening.
• Мне некому звонить по вечерам. — I have no one to call in the evenings (habitually).

So позвонить fits a single completed act better.

Yes, both are dative, but they do different jobs.

• мне = the experiencer, the person in this situation
• некому = the person whom one would call, because позвонить takes the dative: позвонить кому?

So the structure is basically:

• мне — for me
• некому — no one (to whom)
• позвонить — to call

Russian allows this because the two dative forms belong to different parts of the sentence structure.

Because Russian often uses the instrumental case for parts of the day as adverbials of time:

• утром — in the morning
• днём — in the daytime / during the day
• вечером — in the evening
• ночью — at night

So вечером by itself already means in the evening.

You do not say в вечером.

By itself, вечером can be somewhat flexible, and the exact meaning depends on context.

It can mean:

• this evening / tonight
• in the evening

In this sentence, without extra context, many learners can understand it as either:

• I have no one to call this evening
• I have no one to call in the evening

If you wanted a clearly habitual meaning, Russian would often make that more explicit, for example:

• по вечерам — in the evenings

So:

• Мне некому позвонить вечером. — this evening / in the evening
• Мне некому звонить по вечерам. — in the evenings, regularly

No. It means I have no one to call, not no one can call me.

That confusion happens because позвонить takes the dative for the person being called, and there is also мне in the sentence. But here:

• мне is the experiencer: for me
• некому is the missing person I could call

So the meaning is:

For me, there is no one to call in the evening.

If you wanted to say Nobody can call me in the evening, you would need a different structure, for example something like:

• Мне никто не может позвонить вечером.

Yes. Russian word order is flexible, and changing it usually changes focus rather than the basic meaning.

For example:

• Мне некому позвонить вечером. — neutral
• Вечером мне некому позвонить. — emphasizes in the evening
• Некому мне позвонить вечером. — more marked, often literary or contrastive

For learners, the original order is the safest and most natural to use first.