Na obroży jest numer telefonu.

Because the preposition na here means on, and when na expresses location, it normally takes the locative case.

• dictionary form: obroża = collar
• locative singular: obroży = on the collar

So:

• na obroży = on the collar

This is why you do not see the basic dictionary form obroża after na in this sentence.

Obroży is the locative singular form of obroża.

The noun obroża is a feminine noun, and its singular locative form is:

• obroża → na obroży

This form is used after certain prepositions when talking about location, such as:

• na obroży = on the collar
• o obroży = about the collar

Because in Polish, when one noun specifies another noun, the second noun is often put in the genitive case.

So:

• numer = number
• telefon = telephone/phone
• telefonu = of the phone

That means:

• numer telefonu = phone number
  literally: number of (the) phone

This is very common in Polish:

• adres domu = address of the house
• klucz samochodu = car key
• kolor włosów = hair color

Telefonu is the genitive singular form of telefon.

• nominative: telefon
• genitive: telefonu

It appears in numer telefonu because Polish often uses the genitive after a noun to show a relationship like of in English.

Jest is the 3rd person singular form of być (to be) and means is.

So the structure is:

• Na obroży = on the collar
• jest = is
• numer telefonu = a phone number / the phone number

Polish often includes jest in sentences like this to state that something exists or is located somewhere.

So the sentence structure is very similar to English:

• On the collar is a phone number.
• more natural English: There is a phone number on the collar.

Usually, in this sentence, jest should stay.

Polish sometimes omits forms of to be in very informal or special styles, but in a normal sentence like this, jest sounds natural and correct:

• Na obroży jest numer telefonu.

If you remove it:

• Na obroży numer telefonu.

that sounds incomplete in standard Polish.

Polish word order is more flexible than English, so the speaker can choose what to emphasize first.

Starting with Na obroży puts the location first:

• Na obroży jest numer telefonu.
  = On the collar, there is a phone number.

You could also say:

• Numer telefonu jest na obroży.

This version emphasizes the phone number a bit more.

Both are grammatical, but the original sentence is very natural if the important point is where the number is.

Not always. Na has several meanings depending on context, including:

• on
• at
• to
• sometimes other related meanings

Examples:

• na stole = on the table
• na poczcie = at the post office
• na koncert = to the concert

In na obroży, the meaning is clearly on the collar.

A rough pronunciation guide is:

• obroży ≈ ob-RO-zhih

A few helpful notes:

• ż sounds like the s in measure
• the stress in Polish is usually on the second-to-last syllable
• so here the stress falls on bro: o-BRO-ży

And the whole sentence is roughly:

• Na obroży jest numer telefonu
  ≈ na ob-RO-zhih yest NOO-mer te-le-FO-nu

Yes, obroża usually means a collar, especially an animal collar, such as for a dog.

That is why this sentence strongly suggests something like a pet collar with contact information on it.

For clothing collars, Polish normally uses a different word:

• kołnierz = collar (on a shirt, coat, etc.)

So:

• obroża = animal collar
• kołnierz = clothing collar

Polish does not have articles like a and the, so numer telefonu can mean either:

• a phone number
• the phone number

The exact meaning depends on context.

In this sentence, English will often translate it as:

• There is a phone number on the collar.

But in another context, it could also mean:

• The phone number is on the collar.

Context tells you which is intended.

Not normally in this situation. The standard and natural phrase is:

• numer telefonu = phone number

Polish usually expresses this idea with a noun + genitive structure, not with an adjective.

So a learner should remember numer telefonu as the normal expression.