הטלפון שלי בכיס הפנימי של המעיל.

Because in the present tense, Hebrew often uses a verb-less sentence.

So הטלפון שלי בכיס הפנימי של המעיל literally looks like:

my phone in the inner pocket of the coat

but it naturally means:

my phone is in the inner pocket of the coat

If you want to be more explicit, you can add נמצא: הטלפון שלי נמצא בכיס הפנימי של המעיל
= My phone is located in the inner pocket of the coat

Both are correct, but the shorter version is very normal.

שלי means my.

In Hebrew, one very common way to show possession is:

noun + של + pronoun

So:

• הטלפון שלי = my phone
• הספר שלך = your book
• הבית שלהם = their house

So unlike English, where my comes before the noun, Hebrew often puts the possessive part after the noun.

Also, שלי is related to של.
You can think of שלי as something like of me / mine.

Because הטלפון שלי is the most standard, neutral way to say my phone.

In Hebrew, when something is possessed and specific, the noun is often treated as definite, so ה- is very natural:

• הטלפון שלי = my phone
• החבר שלי = my friend
• האוטו שלי = my car

You may also hear forms like טלפון שלי in casual speech, but הטלפון שלי is the safer standard pattern for learners.

Because Hebrew is usually written without vowels.

So בכיס can represent either:

• be-kis = in a pocket
• ba-kis = in the pocket

In this sentence, it is understood as ba-kis because the whole phrase is definite:

בכיס הפנימי של המעיל
= in the inner pocket of the coat

The context makes it clear that this is a specific pocket, not just any pocket.

Because in Hebrew, adjectives usually come after the noun.

So:

• כיס פנימי = an inner pocket
• בית גדול = a big house
• ספר מעניין = an interesting book

This is different from English, where adjectives usually come before the noun.

Because פנימי has to agree with כיס.

כיס is:

• masculine
• singular

So the adjective must also be masculine singular:

• כיס פנימי = masculine singular
• if the noun were feminine, the adjective would usually change form
• if the noun were plural, the adjective would also change

So פנימי is the correct agreeing form for כיס.

Because Hebrew adjectives agree not only in gender and number, but also in definiteness.

So if the noun phrase is definite, the adjective also becomes definite:

• כיס פנימי = an inner pocket
• הכיס הפנימי = the inner pocket

In your sentence, the noun phrase is definite, so the adjective must be definite too:

בכיס הפנימי

Even though the ה- on כיס is hidden by the preposition ב, the adjective still clearly shows the definiteness.

It means of the coat.

So:

• שלי = my / of me
• של המעיל = of the coat

Hebrew often uses של to connect two nouns when English would use:

• of
• an apostrophe-s
• or a possessive structure

So:

כיס פנימי של המעיל
literally = inner pocket of the coat

This is a very common and natural Hebrew structure.

Because this sentence refers to a specific coat, not just any coat.

So:

• של מעיל = of a coat
• של המעיל = of the coat

In context, the speaker means a particular coat, so המעיל is definite.

Yes:

הטלפון שלי נמצא בכיס הפנימי של המעיל

That is also correct and means the same thing.

The version without נמצא is more concise and very natural in everyday Hebrew.
The version with נמצא can sound a little more explicit, formal, or emphatic.

A simple transliteration is:

ha-telefon sheli ba-kis ha-pnimi shel ha-ma'il

A few helpful notes:

• sheli = my
• ba-kis = in the pocket
• pnimi = inner
• ma'il = coat

In ma'il, the apostrophe shows a slight break caused by א.