If your EDGE settings become blank every time you restart the iPhone there is a quick and simple solution to resolve this issue…

Simply delete the file /System/Library/Frameworks/CoreTelephony.framework/Support/UnknownCarrier.plist from your iPhone. Navigate to Settings -> General -> Network -> EDGE and type in your carrier’s APN settings. Restart the phone and your iPhone should now have your EDGE settings permanently saved.

Enjoy.