So without getting to heavy into the details, here’s the lowdown. I have an iPhone 3GS. I am not elligle for an upgrade. My father, who is on the same plan however, IS. I would get the iPhone 4 using his upgrade and he’s willing to take my 3GS. Obviously, since the iPhone 4 uses a micro SIM, I can’t just pop my SIM into that phone. That’s pretty well established. And I’ve been reading on what the best thing to do is. I’ve read this in another topic post:   “I just got the new phone off the truck and rushed to AT&T to re-assign numbers on all the SIM cards (my wife could upgrade, I could not).  BUT DON’T DO WHAT I DID.  The phones come hard-coded with the number that you’ve put in for eligibility of upgrade.  So YOU MUST ACTIVATE THE PHONE WITH THAT NUMBER FIRST.  THEN, you can go to or call AT&T to have the numbers re-assigned to the appropriate SIM cards.  It’s not a problem to re-assign.  They won’t tell you to stay on our original phone or anything.  But, you MUST ACTIVATE WITH ORIGINAL NUMBER, sync it up, THEN you talk to AT&T to get things swapped out. ”   So I’m taking that to heart because the guy has had a lot of problems since he didn’t activate the SIM first. That’s all cool. But I’ve also been reading things here and there that say something like “Be ready to give the wireless number you want to activate, the account holder’s SSN and shipping ZIP code.” and “You can transfer your current phone number to iPhone, or get a new one.” These are both instructions for activating via AT&T’s website as well as through iTunes.   So that got me thinking is this: I will be ordering the phone through Apple’s site. After I receive the phone, during the activation process, is it going to ask me what number I want to use? It’s been forever since I’ve had to activate with my 3GS and I never really paid much attention to it before. If it does, can I just activate it using MY number, making my dad’s old SIM still active, letting us swap without having to visit AT&T? Does anyone know?   UPDATE: After only a few minutes more of snooping (I couldn’t FIND this before), I found THIS for anyone wondering: http://www.wireless.att.com/learn/articles-resources/iphone4upgradesupport.jsp Looks simple enough! Sweet. If anyone still wants to weigh in, feel free though, but I might have just answered my own darn question. Sigh.