ERROR: Could not activate cellular data network: PDP authentication failure

I have an iPhone 3g that was given to me by a friend a couple months ago. Before I got the phone i was on a plan with Softbank in japan with another cell phone. When i got the iphone i unlocked it so that i could use my old sim card with the iphone. everything worked out fine, i was unable to use the internet via 3g since it wasnt on my plan but i could text and make calls with no problem.
Everything was working fine until yesterday when i started getting this error message: "Could not activate cellular data network: PDP authentication failure"
once this happened i could no longer text or make calls. my iphone recognizes the sim card, shows one bar of signal, the network, and the 3g symbol. i have also rebooted and reset the network settings many times.
i have no idea how to fix it, any suggestions would be awesome.

thanks.