iOS: Error message when trying to use a custom Access Point Name (APN)
When activating 3G service on an iOS device using a custom Access Point Name (APN), the error message "Could not activate cellular data network: PDP authentication failure" may appear.
Prior to the release of iOS 3.0, APN user names and password lengths were limited 20 characters for the user name and 32 characters for the password. If your device is not running iOS 3.0 or later, update to the latest version of iOS.
If updating your iOS device is not an option, and you are using a private APN, ask your IT department for a shorter APN user name or password.
If your device is using iOS 4, and your APN was set by a configuration profile, verify with your IT department that the configuration profile was created by iPhone Configuration Utility 3.0 or later. Configuration profiles created by earlier versions of iPCU will not properly set the APN on iOS 4 devices.