WARSAW—The Polish unit of T-Mobile said on Friday it had faced a cyber attack but said its network was not blocked and its critical systems had not been compromised. The company said in a statement it had faced a distributed denial-of-service (DDoS) attack, in which hackers try to paralyze a network by flooding it with unusually high volumes of data traffic. “It was the largest attack of this type on the T-Mobile network, and to our knowledge also the largest direct attack on a mobile network operator in Poland,” it said. T-Mobile also said the attack and its consequences were being analysed and were reported to the relevant authorities.