Learn Hardcore Indonesian: Ketika saya angkat telepon, dia berkata bahwa rapat dimulai lebih cepat. - When I answer the phone, he/she says that the meeting starts earlier.

ketika: neutral and common in standard/formal writing.
saat: also standard; slightly more formal/literary; often seen in news: Saat saya mengangkat telepon, …
waktu: more casual/colloquial: Waktu saya angkat telepon, …

They’re interchangeable here, with register being the main difference.

Is saya angkat telepon correct, or should it be saya mengangkat telepon?

Both are correct.

saya mengangkat telepon is the fully marked, more formal/standard active form.
saya angkat telepon (dropping the meN- prefix) is very common in speech and informal writing. Use the meN- form in formal contexts; the bare verb is natural in everyday conversation.

Does angkat telepon mean “answer the phone”? How does it differ from menjawab telepon or menerima telepon?

angkat telepon / mengangkat telepon: literally pick up the phone; idiomatic for answer a call. Most common.
menjawab telepon: answer a call (focus on replying); also fine.
menerima telepon: receive/take a call, often used for being on the receiving end: Saya menerima telepon dari bos. Related: call someone is menelepon (seseorang) (standard spelling). Colloquial nelpon is very common; nonstandard menelpon is widespread but not the formal standard.

What does bahwa do? Can I leave it out?

bahwa introduces a content clause (that). With berkata, it’s natural: dia berkata bahwa … You have options:

Keep bahwa: Dia berkata bahwa rapat dimulai lebih cepat.
Use direct speech: Dia berkata, Rapats dimulai lebih cepat.
Use a more colloquial verb: Dia bilang (kalau) rapat dimulai lebih cepat. Dropping bahwa after berkata without switching to direct speech can sound off; with bilang, kalau is common in speech.

Why dimulai instead of mulai? What’s the difference?

dimulai is passive: the meeting is begun (by someone/organizers).
mulai is intransitive: the meeting begins/starts. Both are acceptable: Rapat dimulai … and Rapat mulai …. In announcements and formal style, dimulai is very common; mulai is shorter and neutral.

Does lebih cepat mean “earlier” or “faster”? Should it be lebih awal?

Literally, lebih cepat = faster. In everyday Indonesian, mulai lebih cepat is widely used to mean starts earlier (sooner than planned). However, the more precise choice for time is:

lebih awal = earlier Very natural alternatives:
Rapat dimulai lebih awal.
Rapatnya dimajukan. (the meeting was moved earlier)

Earlier than what? How do I say “earlier than scheduled/usual/expected”?

Use a comparator:

daripada jadwal / daripada yang dijadwalkan: Rapat dimulai lebih awal daripada jadwal.
daripada biasanya: Rapat mulai lebih awal daripada biasanya.
daripada yang direncanakan or daripada rencana: earlier than planned.

How do I make clear whether the meeting had already started or would start earlier?

Indonesian uses particles/adverbs, not tense:

Had already started: Dia berkata bahwa rapat sudah/telah dimulai (lebih awal).
Would start earlier (future relative to the call): Dia berkata bahwa rapat akan dimulai lebih awal.
Was moved up (rescheduled earlier): Dia berkata rapatnya dimajukan. Add time words for clarity: tadi, barusan, nanti, besok, etc.

Does dia mean he or she? How can I be specific or polite?

dia is gender-neutral (he/she). To be more specific:

Add context: dia (laki-laki/perempuan) if needed. For politeness toward a respected person, use beliau. In formal writing, ia is also common for he/she.

Do I need an article like “the” before rapat? How do I say “the meeting” vs “a meeting”?

Indonesian has no articles. rapat can be definite or indefinite from context. To be explicit:

The meeting: rapat itu or rapatnya
A meeting: sebuah rapat (or just rapat if context allows)

Can I switch the clause order? What about the comma?

Yes. Both are fine:

Ketika saya (meng)angkat telepon, dia berkata bahwa … (subordinate clause first) — use a comma.
Dia berkata bahwa … ketika saya (meng)angkat telepon. — comma usually not needed.

Is it telepon or telpon/telefon? And is dimulai one word?

Official spelling: telepon (noun), menelepon (verb). You will see telpon/nelpon informally.
dimulai is one word (prefix di-
- verb). Write the passive verb as one word. Write di separately only as a preposition for place: di kantor, di rumah.

Could I use mengatakan or bilang instead of berkata?

Yes:

More formal: Dia mengatakan bahwa rapat dimulai lebih awal.
Informal: Dia bilang (kalau) rapat dimulai lebih awal. If you want to specify the addressee: Dia berkata kepada saya bahwa …, Dia bilang ke saya bahwa/kalau …

Is Rapat mulai lebih cepat also acceptable?

Yes, it’s fine and idiomatic, especially with a comparator:

Rapat mulai lebih awal/lebih cepat daripada jadwal. For formal clarity, many prefer dimulai or lebih awal, or the rescheduling verb dimajukan.