What is a cheap way to call UK from Indonesia?

I want to call the UK while in Indonesia. What's the cheapest way of doing this from a mobile phone?

I can use my UK mobile phone and get a phone card, or would it be better to get an Indonesia phone chip and then a phone card?
Thanks, that's great. I don't think I can use Skype, I specifically want to be able to make calls from my mobile.

I'll look for Esia sim cards. Are they readily available? Where can I purchase them?

cimione_m:
If you get an Indonesian sim card while you are there, that is cheaper than calling from a uk sim card whilst abroad.
When you are 'abroad', meaning your using a uk sim card in another country, it costs you alot more to call anywhere, even to the Uk - and it costs the other person, crazy but true.
But if you use an Indonesian sim card, its just the cost of an international call.

Chaille L:
It would be better if you use Skype.

But if you don't have the time to pull out your credit card and buy a SkypeCredit then buy an Indonesian phone chip then call internationally. (costs about 1,000 IDR per minute. Esia is the cheapest, CDMA, and Telkomsel is the most expensive, GSM).

Tony B:
The cheap way is, DON'T use a mobile phone!

Donny W:
Internet is the cheapest way.
Esia isn't gsm, it's CDMA. You can use satelindo gsm card, in prepaid it called mentari. Or 3 (three) gsm card. You could buy it in many stores in mals, supermarket, bookstore, etc. Hope I could help.

EMSEMSEMS7:
skype