What a difference a few months makes. T-Mobile, which late last year asked the federal government to approve its merger with AT&T (the feds didn’t), is now asking the FCC to block approval of Verizon’s $3.9 billion acquisition of wireless spectrum. The sale, announced in December, would provide Verizon with 122 licenses for wireless airwaves that reach about 259 million people and is designed to bolster what has become a shortage of spectrum owing to the proliferation of mobile services. But T-Mobile argues in a filing today that the deal would allow its rival “to accumulate even more spectrum on top of an already dominant position, while checkmating crucial avenues for growth of its smaller competitors.” The FCC is reviewing the sale, which involved Comcast, Time Warner Cable and BrightHouse Networks selling the mostly unused licenses in a deal that will net them a 60% profit from the investments they made beginning in 2006. Verizon argues that putting that spectrum to use is good for consumers increasingly craving mobile services.