The data includes that belonging to former and potential customers who may be unaware that the company retained their information.
T-Mobile has confirmed that criminals stole the personal information of more than 40 million current and former customers, although it states no financial details were leaked as a result.
The breach was only discovered by the US-based company following reports that criminals were attempting to sell a large database containing its customer data online.
Although T-Mobile cautioned that it had "no indication" any financial information, including credit or debit card details, was contained in the stolen files, it confirmed that personal data was lost.
"While our investigation is still underway and we continue to learn additional details, we have now been able to confirm that the data stolen from our systems did include some personal information," the company stated.
The company's initial analysis indicates that approximately 7.8 million current customers are affected by the breach, as well as 40 million former and prospective customers "who had previously applied for credit with T-Mobile".