If you just need to add minutes you can top up online in a pinch too. I purchased a sim and a cheap phone. I add a small amount of minutes online every 3 months to keep the sim\mins active. Then when I land I'm ready to go with all of my contacts and enough minutes to make all the calls I need.
This should be obvious but make sure friends and family calling your ja number know its an international call. My brother called my ja number frequently one trip and was surprised by the bill.
