How to Unlock your Jailbroken iPhone using Sam, Rejoice locked iPhone owners, we have got you some great news. There is a way to unlock your jailbroken iPhone 3GS, iPhone 4 or iPhone 4S running iOS 5.0 or later, as iPhone-Dev team member MusclNerd confirmed through his Twitter account that he successfully unlocked his iPhone 4S today and got it running on T-Mobile.
A Chinese hacker named Loktar_Sun has manipulating SAM (Subscriber Artificial Module) package and came up with a method to unlock iPhone.

Of course this is great news since you’re waiting AT&T to unlock your iPhone. Now you can, by yourself, unlock your iPhone and use it on other networks.

Once you got the package start following this full guide from here then start using your iPhone on any other network.

How to Unlock your Jailbroken iPhone using Sam Step by step

First you will need to add the following source to Cydia to download the SAM package from Manage > Sourses > Edit > Add, and add the repo. 
repo.bingner.com

Step#1 Install Sam Bingner’s SAM (Subscriber Artificial Module) package. I recommend that you get it from repo.bingner.com since older versions have a different interface and may not work for this purpose.

Step#2 Enter SAM by either going through the settings menu or find the SAMPrefs icon on your springboard. You will need to have the SIM card you intend to use in your phone.

Step#3 Go to utilities and select “De-Activate iPhone”, your ActivationState under “More Information” should now be “Unactivated”

Step#4 With SAM enabled, choose “By Country and Carrier” in “Method”; find your carrier, for some carriers operating more than one Carrier ID you may need to select “SIM ID”; easy to tell since iTunes will not activate if the wrong IMSI is selected.

Step#5 Go to More Information”, copy or write down the IMSI in “SAM Details”, then tap “Spoof Real SIM to SAM”.