A good rule of thumb states that if a company or companies complain about the potential merger of two other competitors (i.e., Sprint complains about AT&T merging with T-Mobile), the merger is likely to increase competition - which benefits consumers - and therefore should be allowed. If consumers complain about the potential merger of two companies, the merger is likely to be anti-competitive and should be prevented.
If AT&T merging with T-Mobile benefits consumers, Sprint has every reason to complain - they will be driven out of business by the economies of scale or enhanced services at better rates. It it fails to benefit consumers, leading to higher prices and increased rents to AT&T, then Sprint can only win by competing and providing consumers more value per dollar than the new merged company.
Given that Sprint has joined the government's suit against the AT&T/T-Mobile merger, I'm guessing that a merger benefits consumers.