Does anyone knows wat is this F931 error?

[Gospelfrances]

Hi, im trying to add the data plan but i got this F931 error at the att website.
I bought my iPhone 3gs from a guy who was trying to reselled at bestbuy and I got it for 120, I don't know if this iPhone was stolen and that's may be the reason for the error.

 

[countrydweller]

I found this using google, you might need to let them know you have a new phone, one compatible with the data plan.

F931: The feature you are trying to add is not compatible with the phone/device on record in the billing system.

 

[Gospelfrances]

I found this using google, you might need to let them know you have a new phone, one compatible with the data plan.

F931: The feature you are trying to add is not compatible with the phone/device on record in the billing system.

Thank you!!!