Updated November 18 at 11:19 a.m. PST to clarify that the data was sold by workers at T-Mobile UK, which is operated separately from T-Mobile USA.
T-Mobile workers sold personal data on thousands of customers to third parties who then called the individuals as their wireless contracts were due to expire, a T-Mobile UK spokesman has confirmed.
T-Mobile notified England's Information Commission, the watchdog agency responsible for safeguarding consumer privacy, and said the activity was done "without our knowledge," according to the BBC.
Information Commissioner Christopher Graham told the news agency his office will prosecute the individuals responsible.
It's the latest black eye for the T-Mobile brand in recent months. (T-Mobile UK and T-Mobile USA are operated separately.)
Last month an outage with T-Mobile USA network left Sidekick users unable to access the Web or their address books for several days.
And earlier this month T-Mobile's network in the U.S. suffered a major outage that left customers unable to send or receive text messages and access voice messages for part of a day. The outage was due to a software error in the back end system that generated abnormal congestion on the network, the company said in a statement.