Я долго не могла дозвониться до сестры, потому что у неё был выключен звук.

Because the subject is я, and in the past tense Russian verbs show gender in the singular.

• я мог = I could, said by a man
• я могла = I could, said by a woman

So this sentence is spoken by a female speaker.

Дозвониться means to get through to someone by phone, not just to call.

It usually implies that:

• you tried calling,
• it was not easy,
• and the goal is to actually reach the person.

So не могла дозвониться до сестры means more than just I didn’t call my sister. It means I couldn’t reach her by phone, often after repeated attempts.

Both are possible, but they emphasize slightly different things.

• не дозвонилась = I didn’t get through
• не могла дозвониться = I was unable to get through

The version with могла emphasizes inability. It often sounds a bit more like I tried, but I just couldn’t manage it. With долго, it also suggests a period of unsuccessful trying.

Because дозвониться requires the pattern:

дозвониться до + genitive

So the case is not chosen freely; it is determined by the verb.

Examples:

• дозвониться до мамы
• дозвониться до врача
• дозвониться до сестры

The nominative form is сестра, and the genitive singular is сестры.

Долго means for a long time.

In this sentence it tells us that the speaker was trying unsuccessfully for quite a while. So the idea is not just I couldn’t get through, but I couldn’t get through for a long time.

The -ся is part of the verb дозвониться. You should learn it as a whole dictionary form.

Here, дозвониться is not just звонить + ся in a simple mechanical sense. The full verb means to succeed in reaching someone by phone.

This is common in Russian: many verbs with -ся develop meanings that are best learned as whole units.

Literally, it is something like:

At her, the sound was turned off

But natural English would be:

• Her sound was off
• Her phone was on silent
• The sound on her phone was turned off

The structure у неё is a very common Russian way to express that something belongs to someone or is associated with them.

Because these mean different things.

• она выключила звук = she turned the sound off
  This focuses on the action.
• у неё был выключен звук = her sound was off / the sound had been turned off
  This focuses on the state or condition.

In this sentence, the important point is not who did the action, but the reason the calls did not work: the phone’s sound was off.

Выключен is a short-form passive participle from выключить.

It agrees with звук, which is masculine singular, so we get:

• выключен for masculine
• выключена for feminine
• выключено for neuter
• выключены for plural

With был, it forms a past-state expression:

• звук выключен = the sound is off
• звук был выключен = the sound was off

Here звук refers to the phone’s sound setting in a general sense. In natural English, depending on context, you might translate it as:

• sound
• ringer
• phone sound
• the phone was on silent

In everyday Russian, выключен звук is a very normal way to say that the phone would not make a sound, so the sister likely did not hear the calls.

Yes, very likely.

The combination of:

• долго
• не могла
• дозвониться

strongly suggests repeated attempts over a period of time. The idea is something like I kept trying to call my sister, but I couldn’t get through, because her phone sound was off.