Do not use SIM card adapters or SIM cards that are modified to fit into the SIM slot. If you need a SIM card of different size, contact your operator.
Try also the following:
- Restart your phone.
- Check that your SIM card is fully inserted.
- If possible, try your SIM card in another phone, or another SIM card in your phone.
- 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.