apn settings

You will only be able to change then on an unlocked phone using an unknown network. Carriers such as Net10 and StraightTalk in the US piggyback on the AT&T and/or T-Mobile GSM networks, and since the base network is known and supported, the APN setting will not be changable. you will have to talk to the carrier in that case about what you need to do to be able to use their network.

What carrier are you trying to use?

Give it a try from your iphone

http://iphone.flexserve.net/select