I just bought a Nokia Lumina 520. I put in the sim card but is says it is missing or damaged. i could have put in the wrong 19 digit sim number but i have no way of telling except the last four digits match.
If you think you've entered the wrong SIM card #, you can try re-entering it via Self Serve. If the number is correct and you're still getting that message, I'd advise you to stop by the store where you purchased it to try a different SIM.
There is a possibility that the SIM reader is faulty on the phone as well, in which case they can exchange the phone for you since it's still within 14 days.