Sharon has a Verizon cellphone but wants to get her old AT&T iPhone 4S added to the plan. Verizon said they can't add that phone to their network, though. Leo says back then, AT&T and Verizon phones only worked with either GSM or CDMA technology. If the phone was newer, like an iPhone 5S or iPhone 6, she could do it because they are called "world phones." She may need to unlock it. But the 4S predates the era of iPhone world phones, and it won't work.
The good news is that Apple still sells the iPhone 5 and 5S and it may be for free with a two year commitment. So that's an option. She may also be able to trade in the old phone for credit.