I received an iPhone 4s from a friend. She used it on AT&T. I had it unlocked by AT&T (at least that is what the emails said). I can't seem to get it to work on Verizon though. I look at the "Carrier" in the Settings/General/About and it says vf nl 24.0. That is confusing me. I am just trying to figure out if I can even use this phone on Verizon or am I just out of luck. I dropped my working phone in chicken stock while making dinner today and I'm just about to cry.
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An iPhone 4s which was used on AT&T does not have the required CDMA radios needed to function on Verizon.
I believe it was not until the iPhone 5(could have been the iPhone 6, though) that all iPhones included the same hardware. Before that, the hardware was different based upon cellular provider.
It was the iPhone 5 that saw Apple standardize and put both GSM and CDMA radios in all iPhones. But prior to the 5, you had to buy the correct model for each network.