I'll agree to adjust my statement. T-mobile has been the biggest driver of changes at the two big boys in the last two years.
T-mobile growth has outpace every other carrier for the past two years. Their revenue growth has beaten both the top carriers for every quarter since Q42013. This past quarter was still the biggest yet. Subscriber growth is up nearly 60% in the past 24 months, there is literally nobody even CLOSE to that.
Granted, when you start at the bottom there is no place to go but up (or flat or dead)..
Every major change that T-mobile has implemented in the past two years, one or both of the majors have had to match. Whether it be the BYOD, no contract for owned phones, large GB family bundles, data roll-over, data bundle doubling, wifi calling, this has all originated at t-mobile..
Indeed, the biggest pressure has been on AT&T, probably more based on phone protocols and footprint and general user type (verizon has more business users and users focussed on reliability as paramount)