I use ATT GoPhone. Purchased the SIM for $10, purchased the per-minute plan with $20 unlimited SMS per month. Will never use minutes so only pay $20 each month. Put the SIM in the GSM shield, go to the ATT website and follow directions. The biggest challenge is getting the ATT mygophone login password to activate the account and SIM. You have to be able to read the SMS messages received because the ATT systems sends a "new temp password" to the SIM/Shield. You cannot complete activation of the card until entering the Temp password.