Samsung Electronics has been dealt another blow stemming from problems with its flagship Galaxy Note 7, as American's second largest telco AT&T stopped all sales of the smartphone over safety concerns. "Based on recent reports, we're no longer exchanging new Note 7s at this time, pending further investigation of these reported incidents," AT&T spokesman Fletcher Cook said in an e-mailed statement on Sunday. "We still encourage customers with a recalled Note 7 to visit an AT&T location to exchange that device for another Samsung smartphone or other smartphone of their choice." Samsung started replacing the Note 7 last month because of a flaw in its lithium battery that can lead to overheating and pose a burn hazard to customers. Airlines have banned customers from using the smartphones on flights, and the evacuation of a Southwest Airlines plane last week was blamed on smoke caused by a replacement device.