1. Restart your phone. 
  2. Check that your SIM card is fully inserted.
  3. Do not use SIM cards that have been modified to fit the SIM slot. If a regular SIM card has been cut to a smaller SIM format you will have to contact your operator to order a replacement SIM.
  4. If possible, try your SIM card in another phone, or another SIM card in your phone.
  5. Your phone may be SIM locked, which prohibits using a SIM card from a different network operator. Contact your network operator to confirm if your phone is locked. They can provide the unlock code and instructions on how to use it.