The big loser in the proposed AT&T/T-Mobile merger (other than consumers) would probably be Sprint, who would end up being the tiny third carrier to the big two of AT&T and Verizon if it goes through. So, unsurprisingly, they're speaking out against it.
In a strongly worded press release, Sprint said that such a merger would "reverse nearly three decades of actions by the U.S. government" in preventing telecom monopolies. They went on to say that "Sprint urges the United States government to block this anti-competitive acquisition." Well, duh.