Summary Box: T-Mobile subs down, iPhone to blame

SUBSCRIBER DROP: Customers have been leaving T-Mobile USA, the country's No. 4 cellphone company, for the last two years. Now that all three of the bigger carriers have the iPhone, that stream has turned into a flood.

THE NUMBERS: It lost a net 526,000 subscribers in the fourth quarter. Worse, it lost a net 802,000 subscribers on contract-based plans, which are the most lucrative.

IN PERSPECTIVE: T-Mobile is now losing subscribers from contract-based plans faster than regular phone companies are losing landline customers.