Many Mobile Virtual Network Operators (MVNOs) such as Straight Talk require that you update the Access Point Names (APNs) on your phone to work with their service. A quick an easy way to do this is to use the Unlockit NZ service by browsing to the following website from your mobile device:
If you are using an iPhone, click on the “Create APN” link at the bottom of the browser window, select your country and carrier, then click on the “Create APN” button. This will create a new “Install Profile” containing the APN settings for your carrier. Just click on the “Install” button and then “Install Now” to update the settings on your phone. Note that you may be prompted to enter your passcode before the updated settings will be installed.
The procedure is similar for Android phones although you’ll likely need to enter these settings manually. You can also just look up the required settings by clicking on the above link from your computer.