No financial data, such as credit card information or social security numbers, was compromised but over 2.3 million customers were affected, says T-Mobile.
The firms say the merger will create jobs and give the US an advantage over China in creating 5G wireless technology.
The company will now focus on smaller "tuck-in" acquisitions, says T-Mobile CFO Braxton Carter.
The US wireless carrier explores taking over rival Sprint in an all-stock deal, after SoftBank offered to give up its majority ownership of Sprint.
Virgin Mobile USA will ditch Android phones in the coming years, claiming it will become the first iPhone-only carrier.
The company targets nationwide 5G coverage by 2020, helped by the airwaves it bought in the US government's spectrum auction.
The firm says it is open to merger talks after a federal ban which barred merger talks among telcos expires this week.
Alan Gow was a co-founder of the original Virgin Mobile in the UK alongside Sir Richard Branson and an architect of the company's economic model as an MVNO.
The US telecom sector could be on the brink of major consolidation under president Donald Trump, says JP Morgan Securities.
We round up trends from across the globe that had us scratching our heads.
Production of Samsung Galaxy Note 7 smartphones has been suspended, following reports of fires in replacement devices.
From 19 July, T-Mobile subscribers will have a year to play Pok mon Go without incurring data costs.