https://www.techdirt.com/articles/20190418/08451142034/doj-isnt-buying-t-mobiles-nonsensical-merger-benefit-claims.shtml

Back in 2011 DOJ regulators blocked AT&T from acquiring T-Mobile, arguing that that the deal would have harmed consumers and resulted in higher rates by eliminating one of just four major wireless players. That's a pretty easy argument to make, given every time a country allows four wireless carriers to morph into three, the sector gets less competitive and prices go up (see: Ireland or Canada). Blocking the deal wound up being a good thing: T-Mobile went on to be even more disruptive, and has helped introduce a number of consumer-friendly market shifts like cheaper international roaming and the death of long-term contracts.

In 2014 T-Mobile and Sprint tried to merge, and regulators (quite correctly) pointed out the deal wouldn't be good for consumers or the market, and blocked it from happening.