IOS – Jailbroken iPhone 3GS shows “No Service” after upgrading to iOS 6.1.6

You can't update your phone to iOS 5.0 Not even if you have your SHSH saved. So nothing you do will allow you to install 5.0 right now. At least for the time being. Your only option is 5.01. The problem is that apple is no longer signing 5.0 and no shsh blobs will help you neither because a new security measure implemented on all 5.X

So if you need an unlock in order to use your phone I would highly recommend not updating to 5.01 because UltraSnow is currently not working on 5.01 despite of what baseband you have. No even with 1.59 baseband (but this is just until ultrasnow is updated to work on 5.01) If you don't need an unlock then you can safely update to 5.01 and do a tethered jailbreak using the latest redsn0w and pointing the ispw to 5.0

And before doing anything please update to the latest iTunes.

I hope this helps.

The message you describe is what happens when a locked phone is restored by iTunes without a SIM present from the carrier that the phone should be locked.

You could use any of the IME I look up tools on the Internet- some are free and some are paid and all make me worry they are harvesting databases of IMEI/serial number/other data in exchange for providing you a service by looking up your phone in the carrier database.

Of course, you can always contact the carrier in question to see what is needed to get service or a Sim card for that device.