Does at&t always let you trade in your old phone for credit?

I saw online were it says trade in your old iphone and get up to 300 dollars credit towards a new phone. Is that only for a certain period of time?

You must sign up for a minimum 2-year contact. TRUST ME - you will end up paying more than you should. Another reason we left ATT and went to Consumer Cellular - our phones were paid up, we get far better service, and pay MUCH less. And more reliable service than ATT, too.