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?
New phone. Only had 2 weeks. Came from Sprint and it is the original Sim Card. Have not removed the back or removed the SIM card. I have only download normal apps, songs and am running lookout.
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.
I might be missing something, but I didn't think the Sprint version of the Optimus G had a SIM card. And, you can't remove the back or otherwise open it up (easily) anyway. - MarkC
My anti-theft program triggered yesterday sending a text saying my sim card was changed. It went on to give the old sim card number. I wondered if you had any details or talked to anyone as to why your problems ocurred?
The Optimus G has an encased back. I've never even removed the battery from it. I'm having the same issue, however it doesn't seem to be affecting my service.