Μόλις δω την αναπάντητη κλήση σου, θα σε πάρω πίσω.

It does often mean just, but it has another very common meaning: as soon as.

So:

• Μόλις έφτασα = I just arrived.
• Μόλις δω... = As soon as I see...

In this sentence, μόλις introduces a time clause and means as soon as.

After μόλις when you are talking about a future event, Greek normally uses the form δω, not θα δω.

So Μόλις δω... means As soon as I see...

A helpful comparison is English:

• When I see it, I’ll call you
  not
• When I will see it, I’ll call you

Greek works similarly here. The first clause refers to the future, but it does not use θα.

Also, δω is perfective: it presents the action as a single completed event, basically once I notice / once I see.

Yes. δω belongs to the verb βλέπω.

This verb is irregular:

• present: βλέπω = I see
• aorist: είδα = I saw
• subjunctive / future-type perfective form: δω = that I see / I will see, depending on context

So in Μόλις δω, the form δω is the expected form from βλέπω, even though it looks very different.

Because after μόλις in this structure, Greek normally does not use να.

You can think of δω here as the same kind of verb form you see in να δω, but after μόλις the να is left out:

• να δω
• μόλις δω

So μόλις να δω would not be natural here.

Literally, it means unanswered call:

• αναπάντητη = unanswered
• κλήση = call

But in normal English, the most natural translation is usually missed call.

So την αναπάντητη κλήση σου is your missed call.

That is a very common point of confusion.

In Greek, σου can function as:

• to you
• your

In την αναπάντητη κλήση σου, it is possessive, so it means your:

• η κλήση σου = your call

This is very normal in Greek. The same weak pronoun form can serve more than one job depending on context.

This is the most natural neutral order:

• την = the
• αναπάντητη = missed / unanswered
• κλήση = call
• σου = your

So literally: the unanswered call your = your missed call

A few points:

1. Greek uses the definite article very often with specific nouns.
   Here it is a specific call, so την is natural.

2. The possessive weak pronoun σου usually comes after the noun phrase:

   • η κλήση σου
   • η αναπάντητη κλήση σου

3. The adjective αναπάντητη normally goes before the noun here.

So the whole phrase is completely standard.

σε means you and is the direct object pronoun.

• θα πάρω = I will take / call
• θα σε πάρω = I will take/call you

In phone language, παίρνω κάποιον often means I call someone.

So θα σε πάρω πίσω means I’ll call you back.

Yes, if you looked at it word by word outside any context, you might think of take back:

• παίρνω = take / get / call
• πίσω = back

But in phone context, παίρνω κάποιον commonly means call someone, and πίσω adds the idea of back.

So:

• θα σε πάρω πίσω = I’ll call you back

This is a very natural everyday Greek expression.

Because θα σε πάρω refers to one single complete action: I’ll call you back.

Compare:

• θα σε πάρω = I’ll call you back once
• θα σε παίρνω = I’ll be calling you / I’ll call you repeatedly or habitually

So πάρω is the right choice for a single callback.

Yes, that would also be grammatical and natural.

But there is a nuance:

• μόλις δω = as soon as I see
• όταν δω = when I see

μόλις sounds more immediate. It strongly suggests the moment I notice it, I’ll call you back.

So in this sentence, μόλις is a very good choice because it emphasizes promptness.