Ako ne vidiš ulaz, pozovi me telefonom.

Ako means if. It introduces a condition:

• Ako ne vidiš ulaz... = If you don’t see the entrance...

This is the normal Serbian word for if in everyday conditional sentences.

Serbian commonly uses the present tense after ako when talking about a real future possibility, just like English often does:

• Ako dođe, javi mi. = If he comes, let me know.
• Ako ne vidiš ulaz, pozovi me telefonom. = If you don’t see the entrance, call me by phone.

So even though the whole situation may be in the future, vidiš stays in the present tense.

Vidiš is the 2nd person singular present form of videti / vidjeti (to see).

So:

• ja vidim = I see
• ti vidiš = you see
• on/ona vidi = he/she sees

Because the sentence is speaking to you singular, Serbian uses vidiš.

In Serbian, ne is usually written as a separate word before the verb:

• vidiš = you see
• ne vidiš = you do not see

This is the standard way to negate most verbs. So ne vidiš literally means don’t see / do not see.

Because ulaz is the direct object of vidiš, so it is in the accusative case. But ulaz is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• ulaz = entrance (nominative)
• vidiš ulaz = you see the entrance (accusative, same form)

This is very common in Serbian.

Ulaz means entrance, entry, or sometimes way in depending on context.

In this sentence, it most naturally means the entrance to a building, courtyard, office, apartment block, and so on.

It can also appear in other contexts:

• glavni ulaz = main entrance
• ulaz u zgradu = entrance to the building

Pozovi is the imperative of the perfective verb pozvati (to call). Perfective verbs are often used for a single complete action:

• pozovi me = call me

By contrast, zovi comes from zvati, which is imperfective and can sound more like keep calling, call in general, or repeated action depending on context.

In instructions like this, Serbian very often prefers the perfective imperative:

• Ako ne vidiš ulaz, pozovi me. = If you don’t see the entrance, call me.

Pozovi is the 2nd person singular imperative of pozvati.

It is used when telling one person to do something:

• pozovi! = call!
• pozovi me! = call me!

If you were speaking to more than one person, you would use:

• pozovite me! = call me! (plural / formal)

Me is the short, unstressed form of the pronoun I / me in the accusative.

• pozovi me = call me

Mene is the long form, used when stressed, contrasted, or after certain prepositions:

• Pozovi mene, ne njega. = Call me, not him.
• Bez mene = without me

In a normal sentence like this, the short clitic form me is the natural choice.

Because Serbian usually does not need an explicit subject pronoun when the verb ending already shows who the subject is.

• vidiš already means you see
• pozovi is already a command to you

So adding ti would usually only happen for emphasis or contrast.

Telefonom is the instrumental singular of telefon.

The instrumental case is often used to express the means or instrument by which something is done:

• pisati olovkom = to write with a pencil
• ići autobusom = to go by bus
• pozvati telefonom = to call by phone / with a phone

So telefonom answers the question how? by what means?

It is correct, but in many everyday situations Serbian speakers would simply say:

• pozovi me = call me
• nazovi me = call me

Because call me already strongly suggests a phone call, telefonom can be optional unless you want to make the method explicit.

So:

• pozovi me telefonom = call me by phone
• pozovi me = call me

Both are understandable, but the shorter version is often more natural in conversation.

Yes. Serbian word order is fairly flexible, although some versions sound more neutral than others.

The original sentence:

• Ako ne vidiš ulaz, pozovi me telefonom.

A possible variation:

• Pozovi me telefonom ako ne vidiš ulaz.

Both mean essentially the same thing. The original version is very natural because it gives the condition first and then the instruction.

It is informal singular, because it uses forms for speaking to one person with ti:

• vidiš
• pozovi

If you wanted the formal or plural version, you would say:

• Ako ne vidite ulaz, pozovite me telefonom.

So the original is what you would say to one friend, one family member, one colleague you address informally, and so on.

It can mean more than one thing depending on context. Pozvati can mean:

• to call someone
• to invite someone
• to summon someone

For example:

• Pozovi me večeras. = Call me tonight.
• Pozvali su nas na svadbu. = They invited us to the wedding.

In your sentence, because of telefonom, the meaning is clearly call me by phone.