Q: how to fix no service on iohone 3gs

Did you already try this?  iPhone: Troubleshooting No Service

1. Toggle airplane mode: Tap Settings > Enable Airplane Mode, wait five seconds, then turn off airplane mode.
2. Turn iPhone off and then on again.
3. Remove the SIM card and verify that it's a valid, carrier-manufactured SIM. Also verify that it isn't damaged, worn, or modified. Then reinsert it.
4. Check for a carrier-settings update. Connect to a Wi-Fi network. Then tap Settings > General > About. If an update is available, iOS will ask you if you want to install it. If Wi-Fi isn't available, connect your device to iTunes.
5. Update your iPhone to the latest version of iOS.
6. Reset network settings by tapping Settings > General > Reset > Reset Network Settings. This will reset all network settings, including Bluetooth pairing records, Wi-Fi passwords, VPN, and APN settings.
7. Restore the iPhone.