You can perform the downgrading procedure that is commonly linked with the iPad 06.15.00 baseband. Get the latest version of RedSn0w, plug in your iPhone, click Jailbreak (don't need to select the IPSW), then there should be an option that says 'Downgrade from iPad Baseband'. Follow the on-screen instructions. Note that even though this "method" has been perfected in terms of how it runs, there are still chances of you bricking the device. For future notice and other readers, DO NOT under ANY CIRCUMSTANCES attempt to restore to a clean iOS while on the iPad baseband, since that's basically asking to throw your phone away (You might as well give it to me if you're going to attempt it)