If there is someone else on the plan who has an active line and currently eligible for a phone upgrade, they can 'gift' you their upgrade. Call customer support to learn more about this option. A lot of families have no idea about this option. In my family, we have a couple people who could care less about new phones every 2 yrs, for example. It has come in handy.

Basically, at minimum you need an unlocked GSM handset which includes the 850/1900 MHz frequency bands for use in North America on ATT's native network. To provider full coverage, choose one with 850/1900GSM & 850/1900 UMTS (3G) frequency bands. For faster data, make sure you have HSPA+ on that unlocked handset. Even better, for access to ATT's fastest/most modern data network, get an ATT issed handset with LTE frequency bands used by ATT.

Any phone with the above characteristics that is ATT issued and still locked would also suffice. In fact, many of the unlocked phones do NOT have the correct frequency bands for use on ATT's network. It's good to go over such purchases in detail before pulling the trigger and buying online.