In Indonesia you will found 4 main mobile operators (“Telkomsel”, “XL”, “Indosat”, “Smart”) with different prepaid cards – for example “Loop”, “IM3” and etc. One of them is more expensive but covers mostly all Indonesia area and you will be reachable while hiking in some volcano or camping somewhere further from civilization. Others will be cheaper but sometimes with delay SMS delivery and etc. Most probably the local people are using the cheapest one and has good connection in their exact area.
I recommend for backpacker using “Telkomsel” operator. It has wide area coverage in Indonesia, better than others. You can sms, call or internet browsing, but the signal depend on location, if it’s far away from transmitter tower, you can not doing internet, just call and sms only.
After you choose mobile operator, sometime you will need to put money to prepaid cards (“pulsa”) each time when you will finish your limits. The “pulsa” amount depends on how much you want to put to prepaid cards. Usually its fixed amount – Rp. 5 000, 10 000, 15 000, 25 000, 50 000 or 100 000 Not in all places you can put bigger amount like 50 000 or 100 000 Rp. As well the “pulsa” validation depends on mobile operator – one suggest validation of 1 month, others – to use till you will finish it (no matter if it will be even couple of months).
*) Today (12/30/2016) i browse ebay.com using Telkomsel internet data, i found pop up ads for a few seconds. Oh, i hate that. World already full of advertisement. I heard it won’t happened if you live in Europe or USA. In here the consumer seems no rights to avoid ads.
**) Today the ads come through SMS. You will accept some SMS as advertisement when you open it. Could be from Restaurant, ‘Donut’, ‘Pizza’ or ‘Cinema’. Much of travelers not love this things. At least you won’t surprised if it happened to you.