Following a large breach earlier this year, some T-Mobile customers appear to have gotten an unwelcome end-of-year surprise.
T-Mobile has reportedly suffered another data breach, a few months after a huge breach in August. The new breach seems to have affected a smaller group of customers, who received notifications of “unauthorized activity” that consisted of hackers checking out customer proprietary network information, pulling off a physical SIM swap, or both, says a Tuesday post by blog The T-Mo Report.
“Customer proprietary network information,” or CPNI, includes all the data T-Mobile has about your phone calls, which, according to the carrier, means “features of your voice calling service (e.g., international calling), usage information (like call logs — including date, time, phone numbers called, and duration of calls), and quantitative data like minutes used.” CPNI doesn’t contain any billing-related information, like names or addresses. 
An unapproved physical SIM swap enables someone else to take over your phone number, and if that person has your password, to potentially gain access to accounts linked to it — such as if you use texts for multifactor authentication. If that’s happened to you, here’s what to do next.
Unlike the earlier breach, which affected more than 50 million customers, this one may have impacted a much smaller number of people.
T-Mobile didn’t immediately respond to a request for comment.
Discover the latest news and best reviews in smartphones and carriers from CNET’s mobile experts.

source