According to our Country Share Tracker (CST) service, T Mobile has overtaken Sprint -- and closed the gap on AT&T -- to become firmly established as the 3rd largest smartphone carrier (by shipments) in the high-value United States market. Despite possible competition from a Google MVNO, device vendors with North American growth ambitions -- such as ZTE, Microsoft (Nokia) or Motorola -- should target T Mobile in 2015. More analysis and data can be downloaded from clients here.