AT&T and T-Mobile announced the US$39 billion acquisition in March, saying the deal could take a year to complete. It faces reviews by the FCC and the Department of Justice. AT&T has said it may have to spin off some assets as a condition of approval.

By requesting to transfer T-Mobile's licenses to AT&T, the companies kick off the FCC's review of the deal. The FCC will consider whether combining AT&T's and T-Mobile's wireless licenses will comply with the agency's regulations. The FCC will also consider how combining the licenses will affect competition.

In the filing, AT&T, the second-largest operator, and T-Mobile are portrayed as facing significant challenges without being able to combine. T-Mobile has "declining market shares and no clear path to Long Term Evolution (LTE), the gold standard for advanced mobile broadband services," the companies wrote.

"AT&T faces network spectrum and capacity constraints more severe than those of any other wireless provider, and this merger provides by far the surest, fastest, and most efficient solution to that challenge," they said.

They go on to assert that the agreement will benefit consumers by reducing the number of dropped calls, boosting data speeds and expanding the deployment of next-generation technologies.