Some three percent of T-Mobile's 77 million customers -- around 2.5 million people -- are estimated to have been affected by the incident, which the company has been unable to explain beyond the assumption the activity was "intentional". This isn't the first time T-Mobile's cybersecurity practices have come under the spotlight. Back in May a researcher discovered a bug on the company's website that allowed anyone to access subscribers' personal details. A spokesperson said that customers affected by the recent attack will be notified soon.