If you happen to restart your phone (iPhone in my case), your SIM might get blocked. Probably this is a setting that comes from mobile operator. If your SIM gets blocked, here is what you should do:
- Do not try to guess your PIN code. Your SIM might permanently blocked after some attempts.
- Find your SIM’s pin code on the back side of the card in which you received your SIM (look for PIN 1, you might have to scratch the card to get the number).