A Cellular One subscriber identity module (SIM) card will work in an AT&T phone, but the phone’s SIM must be unlocked first. AT&T Wireless provides free unlock codes for its customers who meet certain guidelines. Contact AT&T Wireless customer service to determine if you are eligible to receive your free unlock code.
Ya, your Cellone Sim Cards will work in AT&T phones if it a unlocked one.If it is locked also you can unlock it with the help of MobileUnlockSolutions.com where you need to mention your network and country your phone is locked to.They provide you with unlock code and free unlocking instructions.
Once when your phone is unlocked you can use it with any network you prefer.