Thanks for posting.
That could be caused by a few reasons. First, are you attempting to use a different sim card, other than the one that came with the phone?
Has there been any changes to the settings or any new apps that may be attempting to use a GSM network - thus using the sim card?
Any other changes or anything new prior to the issue?
Ok, sounds good. Lets test a couple things if you could..
Would you first go into your settings > More > Mobile network and change the network selection to CDMA only and make sure to turn off all roaming to see if the message comes back?
Also, you may want to try to remove and re-seat the sim card to ensure that it's properly seated in the slot.