If you are trying to use the phone with a different SIM, make sure it is officially unlocked.
If it has been unlocked, follow these steps:
In certain situations, iPhone may present one of the following symptoms:
- "Invalid SIM" or "No SIM Card installed" alert appears intermittently.
- Status bar displays "No Service", "No SIM" or "Searching" in a location with good network coverage.
If you encounter any of the above symptoms on your iPhone, try these steps to attempt to resolve the issue:
- Update your iPhone to the latest version of iOS.
- Toggle Airplane mode On and Off.
- Try turning iPhone off and then on again.
- Check for a carrier settings update. Tap Settings > General > About. If an update is available, a prompt will appear.
- Remove the SIM Card and verify that it is a valid, carrier-manufactured SIM. Also verify that it is not damaged, worn, or modified. Then reinsert it.
- Restore the iPhone.
copied from iPhone: Troubleshooting No Service or No SIM