I'm in a family plan with my stepdad and mother, we have unlimited data and I believe unlimited voice. My mom and stepdad bought there phones a few months before the 3GS launch, they are both eligible for an upgrade. I bought my 3GS on launch date and have the 2/20/2011 upgrade date. My original plan was to use my moms upgrade, and just using my sim card in it. I then figured out the iPhone 4 uses micro sim -__- So then, i said I'll just trim my sim card to use in the iPhone 4, as many have done for their iPad. I'm just not positive if that will work perfectly with voice and data (if it does work perfect that's the route I'm taking) I've also heard, that when you order the phone online and have it shipped that you can have it set up with whichever number (I'm not sure if that's true) If you can, post similar experiences with the 3GS, or other launches. tl;dr Is there any way i can use my moms account, that is eligible for upgrade, to get the iPhone 4 working on my number in our family plan.