Complete speculation, but I'd hazard a guess that they were able to offer such cheap service during the era of near zero interest rates coupled with their maste

Complete speculation, but I'd hazard a guess that they were able to offer such cheap service during the era of near zero interest rates coupled with their masterful use of celebrity to market their service made them an user acquisition machine and that T-Mobile is now paying a mint (no pun intended) to acquire all those users.