iPhone: Using custom APN and unable to use Cellular Data Network services after upgrading to iPhone OS 3.0
After upgrading to iPhone OS 3.0, you may be unable to use some Cellular Data Network services, such as:
Cellular Data connection for Internet services
MMS (Multimedia Messaging Service)
You may also see this error message: "Could not activate cellular data network: You are not subscribed to a cellular data service". This can happen if you edited the Cellular Data Network settings—the APN (Access Point Name) details—either manually or with the iPhone Configuration Utility prior to installing iPhone OS 3.0.
Note: This issue does not affect phone services (making and receiving calls or sending and receiving SMS messages).