I bought an ATT sim at Radio Shack for $25 and it worked
You paid $15 too much. You can pick up one for $10 at ATT and it has $10 worth of minutes on it. ATT also has one for $25 with $25 worth of minutes on it. The minutes on the $10 must be used in 30 days the minutes on the $25 one last for 90 days.