Sprint claims the deal would result in less competition, harm to smaller wireless carriers, and increased costs to consumers:
a combined AT&T and T-Mobile would have the ability to use its control over backhaul, roaming and spectrum, and its increased market position to exclude competitors, raise their costs, restrict their access to handsets, damage their businesses and ultimately to lessen competition.
Sprint's lawsuit follows a similar action by the US Department of Justice last week.
If the acquisition fails to meet regulatory approval, which seems likely based on recent events, it may end up being information accidentally presented to the Federal Communications Commission as part of a filing that ends up derailing it.
In that filing, AT&T admitted it would cost less than $4 billion to expand their LTE network coverage, even without buying T-Mobile.
This contradicts one of their arguments to regulators that they need to purchase T-Mobile for $39 billion, to accomplish the same thing.