This has to be one of the fastest carrier mergers ever. T-Mobile and MetroPCS are now one with each other. The mashing together of the two companies occurred after everything was finalized in a few short months. The newly transformed company will now trade on the stock market under the "T-Mobile US" name and TMUS stock ticker. Here's a quote with a few more details,

The joined entity will operate under the T-Mobile brand, with plans calling for MetroPCS customers to be migrated over to T-Mobile's network by 2015. Once that's done, the spectrum previously devoted to MetroPCS' CDMA network will be repurposed to help build out T-Mobile's LTE network. The goal is to establish a 4G presence rivaling those of larger competitors Verizon Wireless and AT&T. Even as a combined company, the new T-Mobile lags behind Sprint in subscriber count and will maintain its position as America's fourth largest wireless provider at least for now.
It's a shame the Sprint and SoftBank deal is taking longer, but it is probably not too surprising as that one is a bit bigger and has the potential to change the mobile landscape. Still, it's great to see T-Mobile consolidating its power to better compete with the near Duopoly of Verizon and AT&T.

Source: The Verge