Мы позвонили в бюро утром.

Позвонили is the perfective past tense of позвонить, so it presents the call as one completed action.

• Мы позвонили = we called / we made the call
• Мы звонили = we were calling, we called with focus on the process, or we used to call / called repeatedly, depending on context

In this sentence, the speaker is talking about a completed event, so позвонили is the natural choice.

Good question. Russian uses two common patterns:

• позвонить кому? for a person
  
  • позвонить другу
• позвонить куда? / во что? with в or на for an organization, office, institution, or place reached by phone
  
  • позвонить в банк
  • позвонить в офис
  • позвонить на почту

So в бюро means to the bureau / to the office in the sense of calling that organization.

Here бюро is in the accusative case, because after в in this pattern Russian uses the accusative.

However, бюро is an indeclinable noun, so its form does not change. That means nominative, accusative, prepositional, and so on all look the same:

• это бюро
• в бюро
• о бюро

The case is understood from the preposition and the sentence pattern, not from a visible ending.

Because бюро is an indeclinable borrowed noun. A number of Russian loanwords do not change by case, for example:

• метро
• кафе
• пальто
• бюро

Even though the case changes grammatically, the word itself stays the same in form.

Also, бюро is usually treated as neuter.

Утром is the instrumental singular form of утро, and Russian often uses the instrumental like an adverb for parts of the day:

• утром = in the morning
• днём = in the daytime
• вечером = in the evening
• ночью = at night

So утром here functions like a time adverb.

Sometimes yes, but here мы is useful for clarity.

In the past tense, позвонили only tells you that the subject is plural. It does not by itself tell you whether it is we or they.

So:

• Мы позвонили... = clearly we called
• Позвонили... = could mean we called or they called, depending on context

That is why the pronoun is often kept in sentences like this.

No. Russian word order is fairly flexible, and changing it usually changes focus rather than basic meaning.

These are all possible:

• Мы позвонили в бюро утром.
• Утром мы позвонили в бюро.
• В бюро мы позвонили утром.

A sentence-final word often gets a bit more emphasis, so утром at the end can make the time feel like the most informative part.

If you just want a very natural, neutral version, Утром мы позвонили в бюро is also very common.

No. В бюро here means the destination/recipient of the call: to the bureau.

If you want to say from the bureau, you would normally use из:

• Мы позвонили из бюро утром. = We called from the bureau in the morning.

So:

• в бюро = to the bureau
• из бюро = from the bureau

The stress is:

Мы позвони́ли в бюро́ у́тром.

A rough pronunciation guide:

• Мы = mɨ
• позвони́ли = pəzvɐˈnʲilʲɪ
• бюро́ = bʲʊˈro
• у́тром = ˈutrəm

Two useful points:

• In позвони́ли, the stress is on -ни́-
• In бюро́, the stress is on the last syllable

Unstressed о is reduced in pronunciation, so it does not sound like a clear o there.