How to resolve a wrong bill and device charge

We went to wall Mart and enquired about a device with at&t connect but once cerdit check was done the walmart team Said discount is not applicable so we decided not to buy the device with at&t line. But suddenly after a month we got a bill for postpaid line and once we called sutomer care they are saying we have a line and device as well. We never took the device or activated the line. How should we get it solved.