Hi I am trying to put a new sim into my iPhone 3 and it keeps saying 'No Sim' even when i put my old sim back in ? any ideas?

Howdy TriggerPoint96,

Thanks for using the Apple Support Communities.

If your iPhone is displaying "No SIM Card installed" then I'd like you to please follow the helpful steps outlined below.

1. Make sure that you have an active plan with your carrier.
2. Update your iPhone to the latest version of iOS.
3. Turn airplane mode on and then off again.
4. Restart your iPhone.
5. Go to Settings > General > About and check for a carrier settings update.
6. Remove your SIM card from the SIM card tray and then put the SIM card back. Make sure that the SIM tray closes completely and isn’t loose. If you use a SIM tray from a different model iPhone or from another phone manufacturer, it might not fit properly.
7. Try using another SIM card. If you don't have one, go to your carrier's retail store and ask them to test with another SIM card. You might need to replace your SIM card.

If you still see an alert about your SIM card, contact Apple support.

Learn what to do if you see a SIM not supported alert.

If you get an alert about the SIM card in your iPhone - Apple Support

Have a good one.