it should just say no service. it gets carrier alpha tag info from the tower. if theres no connection to the tower to get that info, it wont be displayed. however, under the about screen in settings, it will read the carrier info there since it recognizes the SIM carrier. but what appears on the screen itself only displays current connection info.
for example, i put an old deactivated att sim card in my verizon iphone 5. the screen said "Searching" for a few minutes and eventually changed to "No Service". in the about screen, the carrier info changed from Verizon to At&t though.