Making calls from the prepaid or rental is expensive. If I have to make an international call, I use Skype or MagicJack. If I have to make a domestic call, I wait until I have access to a household landline, Skype if I'm at a hotel, or for short calls use a payphone with an NTT prepaid card.
You can make international calls at the rate of local 03 calls.
http://www.geteazy.com/mltel/gjdhe.html