Christie was given an iPhone 5S with AT&T for a gift and she uses Sprint. How can she use it? Leo says there's an issue with wireless frequencies, so she can't use it on Sprint. But she may be able to use it with T-Mobile. Leo uses T-Mobile in Northern California in urban areas.
Another option is to go with a mobile virtual network operator (MVNO). Google has one called Google Fi that uses both Sprint and T-Mobile, and it's $20 for voice and text and $10 per GB of data. She would need an invite, but she can request it. It'll use both Sprint and T-Mobile depending on what's best. Leo says she could try before she buys for a few weeks.
But if it works well in her area, then Leo suggests moving to T-Mobile.