Contact your carrier and ask them for the settings then enter them manually. After upgrading to iOS 6 mine defaulted back to what they were before I changed them but I still had the SMS message from the carrier with the new settings in, so just changed them back.
I am in afghanistan and have verizon. I was told that in order for it to "Work" it has to be reactivated on the network using the verizon towers. Not sure who your carrier is but all I know is I am pissed! I hope this helps.