could not activate cellular data network

[dangre]

I just bought an iphone 3gs on ebay and can make calls through my carrier but can't get a data connection through it. It gives me the "could not activate cellular data network, you are not subscribed to a data service". My service provider, Sasktel says I have a data plan connected to this phone. They told me to contact apple for support. The phone was on ATT before and I unlocked it myself. I have reset the network, rebooted, entered the cellular data network info myself, used the "unlockit.co.nz" service but can't get it to connect to 3g data plan. It works on wifi. The version is 4.0.2, firmware 05.13.04. Any ideas?