SIM blocked after 3 wrong PIN
To protect your phone, your SIM card is blocked after 3 wrong PIN codes. Therefore, you will need a PUK code to unblock the SIM card. You can obtain a PUK code here. Once you find your PUK, please type it on your phone to unblock it. If you type your PUK code incorrectly for 10 times, your SIM will be permanently blocked so you will need a new SIM card.
SIM blocked for inactivity
If your phone number is inactive during a period of 6 months, your SIM card will be deactivated. If you want to prevent this deactivation, you need to use your phone from time to time.
SIM blocked after abusive usage
If you happen to use your SIM card for a usage which is forbidden by the Terms and Conditions (e.g. tethering with an unlimited goodybag) the service may be suspended.