zdnet[.]com/article/t-mobile-confirms-sim-swapping-attacks-led-to-breach/
T-Mobile won't say how many people were effected.
Some had their customer proprietary network information (CPNI) leaked, others had their SIMs swapped and a small group suffered from both. CPNI includes information about a customer's plan, the number of lines, the phone numbers, the billing account and more.