My samsung galaxy core prime is kind of old. It has the same sim card in it. But recently I get locked out suddenly. Then the error code is on the lock screen saying sim is locked, there is no network and emergency calls only. I can get it unlocked by putting in my phones pku codes (which I had to wait on the phone for an hour just to talk to someone who knew what they were doing! Would a defective sim card make it lock like that?
BTW I'm curious, do you still have the package that the SIM came with? If you do, can you check if PUK code is there?