Before you start

The passport you entered Indonesia on (checked at registration and, if needed, at customs IMEI registration)
An unlocked phone that accepts a physical nano-SIM or supports eSIM
Your phone's IMEI number (dial *#06#) if you plan to stay beyond ~90 days
A valid entry stamp or visa (VoA / e-VoA / KITAS) — required for customs IMEI registration

Step-by-step

1
Pick your carrier — default to Telkomsel
Telkomsel is the only operator present in every province and has the strongest signal on ferries, smaller islands (Nusa Penida, the Gilis) and rural areas — the safe all-round choice. XL Axiata and Indosat (IM3) are cheaper and fine across Canggu/Seminyak/Ubud and Java, but coverage drops off the tourist corridor. If you'll travel beyond south Bali, Telkomsel is worth the premium.
In personWho: YouFree to decide
2
Buy the SIM at an official store or supermarket, not the airport
Get a Telkomsel SIM at a GraPARI store (Kuta, Denpasar, Ubud) or a supermarket, where 25GB/30 days runs roughly IDR 100,000-150,000. Airport kiosks charge IDR 250,000-350,000 for similar data — a steep markup. Alternatively pre-order the Tourist card on Telkomsel's website and collect it on arrival.
In personWho: You15-20 min in store~IDR 100,000-150,000 official / IDR 250,000-350,000 at airport
3
Register the SIM against your passport (mandatory)
Under Indonesia's SIM-registration rule, every prepaid SIM is inactive until registered. The store clerk scans your passport and links it to the number; locals instead use their NIK plus biometrics. One passport can register at most three numbers per operator. Buying in person is easiest because staff complete this for you on the spot.
In personWho: You (the clerk does the filing)Activates within minutes to ~1 hourIncluded in SIM price
4
Handle the IMEI rule if you're staying long-term
A foreign-bought phone works on local networks for about 90 days, then is blocked at the device (IMEI) level — buying a new SIM won't fix it. For a longer stay, register the IMEI at the customs (Bea Cukai) desk on arrival at the airport: it's free for devices valued under USD 500. Register late and customs may assess import tax on the phone's full value. Tourist SIM packages come with a temporary pre-validated IMEI profile, which is why short-stay visitors rarely notice the rule.
In personWho: You (at airport customs)Best done on arrival; ~30 min at customsFree under USD 500; import duty if over the threshold or done late

Documents you’ll need

Passport (photo page) — to register the SIM and for customs IMEI registration
Active visa or entry stamp (VoA / e-VoA / KITAS) — for IMEI registration
Phone IMEI number (dial *#06#) — for customs registration if staying beyond ~90 days
Local proof of address or KITAS — only needed for a postpaid/long-term contract, not a prepaid SIM

Things most newcomers don’t know

The airport SIM kiosks are a tourist-priced trap — you pay 2-3x for the same data.

Airport stalls charge around IDR 250,000-350,000 for 18-25GB, while a GraPARI store or supermarket sells comparable Telkomsel data for ~IDR 100,000-150,000. If you have Wi-Fi or roaming on landing, buy in town or pre-order online.

Source: simcardairportbali.com / Telkomsel

The thing that actually strands long-stayers is the IMEI rule, not the SIM.

The block is tied to your phone's hardware IMEI, not the number, so swapping SIMs does nothing. A phone brought from abroad goes dark after ~90 days unless you register it at customs — do it on arrival while it's free (under USD 500) rather than later when tax can apply.

Source: backindo.com / gayabalivisa.com

Default to Telkomsel if you'll leave Bali's south; pick XL/Indosat only for budget urban use.

Telkomsel is the only carrier in every province and dominates rural areas, ferries and outer islands. XL and Indosat are cheaper and fine in Canggu, Seminyak, Ubud and Java, but their signal thins out fast off the main tourist routes.

Source: unusualnomad.com

eSIM is a clean way to stay connected on day one without a kiosk visit.

Telkomsel and Indosat offer local eSIMs, and travel eSIMs (Airalo and similar) work on arrival with a tourist IMEI profile — handy for the first weeks before you sort a long-term physical SIM and IMEI registration. Confirm your phone supports eSIM first.

Source: esim.balieasy.com / backindo.com

Common mistakes to avoid

Buying from a random street stall that registers the SIM under someone else's ID — if it's not tied to your passport you can lose the number, and you've wasted one of your three-per-operator slots.
Assuming the phone you flew in with will work indefinitely — it's blocked by IMEI after ~90 days; settle the customs IMEI registration early if you're moving long-term.
Registering the IMEI late — you may be charged import duty on the phone's full value instead of registering it free under the USD 500 threshold.
Overpaying at the airport, or buying a cheap local-only SIM and then road-tripping to Nusa Penida or the Gilis on a non-Telkomsel network and losing signal.