Instructions on how to jailbreak and unlock your iPhone 3G on iOS 4.2.1 using PwnageTool for Mac. This tutorial will explain how to update your baseband if needed for unlock.
If you updated your iPhone 6 or iPhone 6 Plus to iOS 8.0.1 you may be experiencing cellular service or Touch ID issues. Here's the instructions on how to downgrade back to iOS 8.0.