Cheapest way to go, I bought a similar go-phone when my old att phone was consumed by child. Insert sim card from current phone and use.
http://www.bestbuy.com/site/AT%26T+GoPhone+-+Nokia+2320+No-Contract+Mobile+Phone+-+Black/9502549.p?id=1218115833245&skuId=9502549&st=att&cp=1&lp=4