I've been using Yahoo Messenger for chatting with my friends, working partners, and families. If no internet connection is present, you can try using it on your mobile phones. There are a few options such as Yamee, YmTiny, YaGo, eBuddy, etc. I haven't tried them all. The only one I tried is Yamee. And IMO so far Yamee is the best and free yahoo messenger application for mobile phone. You can download Yamee (the latest version I used is version 1.3) in here .
I have tried it on several phones, such as Nokia 6270 (GSM), Sony Ericsson K750i (GSM), and Nokia 2865 (CDMA) and it worked perfectly. Yamee supports a lot of mobile phones. You can check it on the above link. Here is the screenshot I captured from above link.
For GPRS connection settings in Indonesia, you can check these links :
Indonesian Operators GPRS tariff ( XL, Telkomsel , Indosat , Axis):
Currently, AFAIK, the cheapest GPRS tariff is AXIS, the most expensive is XL.