I have a iphone 3g 8GB at&t is it possible to switch phone to cricket?

When I take the at&t SIM card out and the check the carrier on settings it says not available so i'm wondering maybe I could put a cricket SIM instead or flash it can someone help me.

Contact att first because the phone is most likely lock then ask them to unlock it so it can be used with any carrier along as they are a cdma carrier meaning they us sim cards