Here’s a common error that many iPhone 3GS users are complaining about. They got “No Service” on iPhone 3GS after jailbreak and unlock. You have to know that no service on iPhone 3GS mainly occurred due to GPS failure after upgrade iPhone 3GS to 6.15.00 baseband.
Few tips that may help you to fix no service on iPhone 3GS after jailbreak and unlock :
Tip 1: Install current ultrasn0w version – Ultrasn0w 1.2.3
Tip 2: Remove then reinstall ultrasn0w : Open Cydia, tab on search then type ultrasn0w > Modify > Remove , then search again for ultrasn0w and install it.
Tip 3: Reset your network settings : Settings > General > Reset > Reset Network Settings
Tip 4: Eject your SIM card then turn OFF your iPhone. After few minutes, turn on your iPhone then re-insert your SIM.
Tip 5: Re-jailbreak and re-unlock your iPhone by restoring to pre-jailbroken custom firmware using Sn0wbreeze or Pwnagetool -