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.

Breakdown of Telepon saya di tas; sinyalnya kembali setelah saya mengisi ulang pulsa dan keluar gedung.

• adalah: to be
• saya: I
• di: in
• dan: and

Questions & Answers about Telepon saya di tas; sinyalnya kembali setelah saya mengisi ulang pulsa dan keluar gedung.

Do I need ada in Telepon saya di tas? Is Telepon saya ada di tas better?

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.” is also fine and sounds a bit more general; points to a specific, known signal.