Question - if I buy a used 16 gig iPhone from someone do I need to get a new SIM card to make it work with my current mobile #? Ultimately here is what I want to do; 1) Take my current 4 gig iPhone and transfer it to my wife 2) Get a used 16 gig one and make it mine I believe I can take my sim card and put it in the 16 gig phone and that will then work as my current # - correct? If not please correct me. Then my original question still exists, how do we get the 4 gig iPhone to be my wife's #? Thanks!