How can I get Verison phone to work on AT&T?
I accidentally gave a Verizon phone to my 10 year old niece, who is 23 years younger than me. There is no SIM inside, and it is an iPhone 4s 16GB Black (MD276LL/A). I don't want to lose data, but she really wants a phone number since she uses AT&T Nokia Lumia 635, I forgot about the calling, and forced her to switch to Verizon. How can we unlock and switch the SIM Card so that it can work with her current number? Or I need to buy an iPhone 6?
iPhone 6s Plus, iOS 9.3.2, Rose Gold, 128GB, AT&T Contract