These are cell phones which are amplified and are specially designed for seniors. An amplified "unlocked" cellphone can be used on any GSM network that uses a SIM card. In the United States, AT&T and T-Mobile are GSM providers. If you purchase an unlocked phone you'll be able to use either provider - rather than purchasing a phone from them which is locked to a particular network. A GSM world phone supports all four GSM frequencies and can be used virtually anywhere in the world, which allows you to take advantage of local rates rather than being charged for roaming, by using a prepaid SIM card.