Begitu telepon berdering, saya segera mengangkatnya.

In this sentence, begitu means as soon as.

So Begitu telepon berdering, ... means As soon as the phone rang / started ringing, ...

A useful thing to know is that begitu has more than one meaning in Indonesian. In other contexts, it can mean like that or that way. But at the beginning of a clause like this, it often means as soon as.

Berdering means to ring.

So:

• telepon berdering = the phone rings / is ringing / rang

The prefix ber- often makes a verb that describes a state, action, or activity associated with the base word. Here, the base is dering, which is related to the sound of ringing, and berdering means to produce a ringing sound.

You do not need to translate ber- by itself every time; it is better to learn berdering as a complete verb meaning to ring.

Both are possible, and both can mean the phone rings.

There is a small nuance:

• berdering is specifically to ring
• berbunyi is more general: to make a sound / to sound

So:

• Begitu telepon berdering... = As soon as the phone rang...
• Begitu telepon berbunyi... = As soon as the phone made a sound / rang...

For a phone, berdering often sounds a bit more specific and natural because phones traditionally ring.

Here mengangkatnya means to answer it or to pick it up, referring to the phone.

Literally, mengangkat often means to lift / raise / pick up. With phones, Indonesian commonly uses angkat or mengangkat to mean pick up the phone, which naturally extends to answer the phone.

So in this sentence:

• saya segera mengangkatnya = I immediately answered it / picked it up

Even though the literal idea is lift, the natural meaning here is answer the phone.

The -nya refers back to telepon.

So:

• mengangkat = to pick up / answer
• mengangkatnya = to pick it up / answer it

In English, we often repeat or replace the noun with it, and Indonesian can do the same with -nya.

This makes the sentence flow naturally:

• Begitu telepon berdering, saya segera mengangkatnya.

You could also say:

• Begitu telepon berdering, saya segera mengangkat telepon itu.

But that sounds more explicit and a bit less smooth in this context.

Yes, you can.

• saya segera mengangkat telepon = I immediately picked up the phone
• saya segera mengangkatnya = I immediately picked it up

Both are grammatical. The version with -nya sounds more natural when the phone has just been mentioned, because Indonesian often avoids repeating the noun unnecessarily.

So mengangkatnya is a very normal choice here.

Segera means immediately / right away / promptly.

In this sentence:

• saya segera mengangkatnya = I immediately answered it

Its position here is very natural. Indonesian adverbs like segera often go before the verb:

• saya segera mengangkatnya

You may also hear langsung in everyday speech:

• Begitu telepon berdering, saya langsung mengangkatnya.

Both are fine, but langsung is often more common in casual conversation.

The comma separates the two parts of the sentence:

1. Begitu telepon berdering
2. saya segera mengangkatnya

The first part gives the time condition: as soon as the phone rang The second part gives the main action: I immediately answered it

So the comma works much like it does in English after an introductory clause:

• As soon as the phone rang, I immediately answered it.

Indonesian verbs usually do not change form for tense the way English verbs do.

So:

• berdering
• mengangkatnya

do not themselves tell you whether the action is past, present, or habitual.

The time is understood from context.

This sentence could mean:

• As soon as the phone rang, I answered it immediately. (past narrative)
• As soon as the phone rings, I answer it immediately. (habitual)
• As soon as the phone rings, I’ll answer it immediately. (future-like, depending on context)

If the surrounding context is a story, learners will usually understand it as past.

Here telepon most naturally means the telephone / phone as the thing that rings.

So:

• telepon berdering = the phone rings

In some contexts, telepon can also relate to a phone call, but in this sentence the meaning is clearly the device or phone that is ringing.

Because Indonesian commonly uses Subject + Verb word order, just like English.

So:

• telepon berdering = the phone rings
• saya mengangkatnya = I answer it

This makes the sentence very straightforward structurally:

• Begitu telepon berdering, saya segera mengangkatnya.

= As soon as [the phone rang], [I immediately answered it].

It is natural and correct, and it sounds fairly neutral to slightly formal.

A few style notes:

• saya is neutral and polite
• segera is a bit more formal than langsung
• mengangkatnya is standard and natural

In casual spoken Indonesian, someone might say:

• Begitu telepon bunyi, saya langsung angkat.
• Begitu telepon berdering, saya langsung angkat.

These are less formal and more conversational.

Your original sentence is a good standard written Indonesian sentence.

Yes. Some possible alternatives are:

• ketika = when
• saat = when
• sesaat setelah = a moment after
• setelah = after

But they are not all identical.

For example:

• Begitu telepon berdering, saya segera mengangkatnya.
  = As soon as the phone rang, I immediately answered it.

• Ketika telepon berdering, saya segera mengangkatnya.
  = When the phone rang, I immediately answered it.

Begitu emphasizes the immediate sequence more strongly than ketika. So it is a very good choice here.