If you are planning to upgrade your iDevice to iOS7, you can prevent your internet settings from being reset.
While your phone is updating, you will be asked if you want to udpate your carrier settings. If you decline, the upgrade will go on, and your current Internet and MMS settings will be kept. That's the easiest option.
If you chose to update your carrier settings during the upgrade, the 48 settings will have been removed, and you will need to set them up again manually. We'll tell you how to do it in the next section.
When you are upgrading from one major version of iOS to another, your Internet and MMS settings may be reset, so you need to set them up again.
When trying to access an Internet page, if you see a message reading "Could not activate mobile data network: You are not subsribed to a mobile data service", it means you need to re-enter your Internet settings.