Mobile-8 has changed it's tariff for their prepaid products. I just got a confirmation SMS message that now they have changed their tariff to IDR 20/seconds to other operators. They also use IDR 25/seconds rate if you calls to home numbers (local/non local). Since they use per second rate, so you will only pay what you have used. For example, if you spend 25 seconds, then you will get charged 25x25 = IDR 625, not being rounded to 30 seconds again like before.
For more information on Fren's tariff, please see their official home page.