My mom is across the country using AT&T on a separate account than me and is eligible for an upgrade. She is happy with her flip phone though and has no interest in phones (just uses it as an emergency phone on a family plan with my dad and sis). She has given me permission as an authorized user to use her upgrade on her account to get an iPhone 5. When I get the iPhone 5 at an Apple Store under her account, can I just enter my phone number and activate the phone to my AT&T account? Or do I "have" to activate it using her number? But if I activate it using her phone number, won't her flip phone stop working and I'll still be unable to do a SIM-switch since its a nano simcard? Please guide me on how I can keep her flip phone to continue working and I can use my number on the iPhone 5 I'll get by using her upgrade eligibility. Thanks.