Kerry FlynnA T-Mobile store in New York. Photo: Gabby Jones/Bloomberg via Getty ImagesT-Mobile has agreed to acquire Vistar Media, an ad tech firm specializing in digital out-of-home, for $600 million, the companies announced on Monday.Why it matters: The all-cash deal supports the telecom's growing ad business as it seeks to generate revenue beyond phone and internet plans.Zoom in: T-Mobile said the deal combines Vistar's tech capabilities and scale with its customer insights and data. New York...