It depends if your iPhone is capable of using the iPad baseband. Read the latest blog post on the iphone dev teams blog and you'll find helpful information regarding this issue. Also, there used to be a program that will tell you if your phone can handle the iPad baseband. Either way, first confirm that your phone can upgrade to 6.15. Triple check everything first before actually doing anything because if you make the wrong assumption you can fry your chip which turn your iPhone into an iPod.