How do you get service on iphone 3g after switching sim cards with another phone?

To get service on your iphone 3G after switching SIM cards from another phone, insert the new SIM card and restart the iphone. Ensure that the SIM card is activated and compatible with the iphone. If prompted, follow any on-screen instructions to set up your network settings. If there are issues, you may need to contact your carrier for assistance with activation or compatibility.