Here are the steps that has to be follow in-order to activate iPhone 3G, 3GS without the need of the original Sim Card.
In-order to follow this, first you are require to download the latest Redsn0w 0.9.6b. Remember that you must have a jailbroken iPhone if you want to activate your iPhone without the use of the original SIM.
The first steps require you to click to open Cydia from the springboard.
Tap the source you have just added or can tap on search and find “Bingner.com repo” once found look under S for SAM and SAMPrefs
Again start the latest Redsn0w 0.9.6b6.
Select “Deactivate” option as shown in the screenshot below, which can led your iPhone to revert back to a stock lockdown in order for SAM to function.
Start iTunes. Make sure your iPhone is connected. Allow iTunes to activate your iPhone officially.
Once done, you can see the following message on your iPhone.
Now simply follow the steps posted here to unlock your phone using Ultrasn0w 1.2 so that you can use it on any carrier you like.