Learn Hardcore Indonesian: Telepon saya di tas; sinyalnya kembali setelah saya mengisi ulang pulsa dan keluar gedung. - My phone is in the bag; the signal came back after I topped up the credit and left the building.

Both are correct. Without ada is natural and simply states location. Adding ada emphasizes existence/availability or contrast. For short answers, Ada di tas is very common.

Should it be di dalam tas instead of di tas?

Both are fine. Di tas already implies “in the bag” because a bag is a container. Use di dalam tas if you want to stress “inside,” or to avoid any chance of confusion with di atas tas (“on the bag”).

Is telepon the right word for a cell phone? What about HP, ponsel, or handphone?

HP/hape: most common in everyday speech for a cell phone.
telepon: generic “telephone”; can mean a cell phone, but some may think of a landline.
ponsel: formal/neutral for “mobile phone,” common in writing.
handphone: widely used in speech; less in formal writing. All work here; the most natural casual choice is HP.

What does -nya do in sinyalnya? Could I say sinyal kembali instead?

-nya marks possession (“its/his/her”) or definiteness (“the”). Here it reads as “its signal” (the phone’s signal) or “the signal.” Sinyal kembali is also fine and sounds a bit more general; sinyalnya kembali points to a specific, known signal.

Is kembali the best way to say the signal came back? What about lagi, balik, or muncul lagi?

All work, with register differences:

kembali: neutral and clear.
muncul lagi / ada lagi: casual and very common.
balik: casual; sinyalnya balik is informal. Avoid bare sinyal lagi; add a verb like muncul or ada.

Are setelah and sesudah interchangeable? What about habis/abis?

Setelah and sesudah are interchangeable standard forms. Habis/abis is colloquial. All mean “after.” Example: Sinyalnya kembali sesudah saya mengisi ulang pulsa...; Sinyalnya balik habis saya isi pulsa... (casual).

Is mengisi ulang pulsa the only way to say “to top up”? Could I use mengisi pulsa or isi ulang pulsa?

All are common:

mengisi ulang pulsa: “refill credit,” slightly more explicit.
mengisi pulsa / isi pulsa: shorter, very common.
isi ulang pulsa: used as a noun (“a top-up”) or as a casual verb phrase. Casual speech often has ngisi pulsa.

What exactly is pulsa?

Prepaid mobile credit in Indonesia for calls and SMS, and sometimes data if you don’t have a data package. For internet, people often buy paket data/kuota. Note that pulsa is not battery power.

Should it be keluar gedung or keluar dari gedung?

Both occur. Standard/clearer Indonesian uses keluar dari gedung (“exit from the building”). Keluar gedung is common in speech and headlines but can feel clipped in formal writing.

Why is the subject omitted in dan keluar gedung? Should it be dan saya keluar gedung?

Indonesian often drops repeated subjects when context is clear. Here, saya is understood for both actions. Dan saya keluar (dari) gedung is fine if you want extra clarity or emphasis.

Can I change the clause order?

Yes. All are natural:

Sinyalnya kembali setelah saya mengisi ulang pulsa dan keluar dari gedung.
Setelah saya mengisi ulang pulsa dan keluar dari gedung, sinyalnya kembali.
Telepon saya di tas. Sinyalnya kembali setelah... (using a period instead of a semicolon)

Is the semicolon necessary here?

No. It’s acceptable, but everyday Indonesian more often uses a period: Telepon saya di tas. Sinyalnya kembali... A comma alone is less standard unless you add a conjunction.

Should it be di tas or di tas saya?

Both are correct. Di tas works when the bag is understood from context (e.g., the one you’re carrying). Di tas saya explicitly says it’s in your bag.

Is saya the best pronoun here? What about aku, gue/gua, or the -ku ending?

saya: neutral/formal, safe with strangers.
aku: informal/intimate.
gue/gua: very casual Jakarta slang. You can also attach -ku to nouns: teleponku/HP-ku di tas. Match the pronoun to the social setting.

Does mengisi ulang pulsa imply I topped up before or after leaving the building? Does the order matter?

It just says both actions happened before the signal returned; order isn’t forced. If order matters, make it explicit:

First top up, then leave: Setelah saya mengisi ulang pulsa lalu keluar dari gedung, ...
First leave, then top up: Setelah saya keluar dari gedung lalu mengisi ulang pulsa, ...

Do I need to say pulsa saya (my credit)?

Not usually. Mengisi (ulang) pulsa normally implies your own account from context. Add pulsa saya or pulsa HP saya only if you need to disambiguate.

Is the spelling telepon or telpon?

Standard Indonesian uses telepon. Telpon is a common informal spelling in chats but is nonstandard.