Many iPhone 3GS users have been complaining about getting No Service Error after unlocking it from Ultrasn0w after Jailbreak. Geohot has reported that its was due to GPS failure after upgrading the device to 6.15.00 baseband but the same can be fixed using procedure below,
- Install Ultasn0w 1.2.3
- After installation is complete remove by visiting Cydia and searching for Ultrasn0w > Modify > Remove,
- Now Re-Install Ultrasn0w by searching Cydia again for ultrasn0w and install it.
- Reset your network settings by visiting Settings > General > Reset > Reset Network Settings
- Now Remove/Eject your SIM card and turn OFF your iPhone
- Wait for a few minutes and turn on your iPhone without SIM
- When the iPhone has turned on Re-insert your SIM card
Note: If still the issue is not resolved try restoreing your device to pre-jailbroken custom firmware using SnowBreeze or PwnageTool then Re-jaibreak using Redsn0w Jailbreak or JailbreakMe and Re-unlock it using Ultrasn0w.