In Short

T-Mobile is the multi-national carrier subsidiary of Deutsche Telekom AG that serves iPhone and iPad owners, among others, in Europe and the United States.

T-Mobile US, Inc. is the American operating entity of T-Mobile International AG. The fourth-largest carrier in America, T-Mobile U.S. — NYSE: TMUS — is headquartered in Bellevue, Washington. The company has 49.1 million customers across the United States and has annual revenues about $24.42 billion.

T-Mobile's roots go back to the mid-1990s, when VoiceStream Wireless PCS was created by Western Wireless Corp. The business spun off in the late 1990s, and T-Mobile acquired it and renamed it in 2002. T-Mobile developed a niche for itself with the Danger Hiptop, rebranded as the T-Mobile Sidekick — an early smartphone with a slideout screen and a QWERTY keyboard that proved popular with wireless users interested in messaging, email and other online access. Microsoft acquired Danger in 2008 then in 2009 its servers suffered a massive outage that made many Sidekick users lose their data. T-Mobile lost customers in droves.

Rival wireless company AT&T attempted to acquire T-Mobile in 2011 in a $39 billion stock and cash offer. After facing heavy resistance from the United States government, AT&T withdrew its offer. T-Mobile then merged with MetroPCS Communications, then the nation's sixth largest wireless carrier. The combined company, T-Mobile US, began trading on the New York Stock Exchange as TMUS.

Since then, T-Mobile has embarked on an aggressive strategy to modernize and expand its wireless network. The company operates a wireless network that communicates using Global System for Mobile communications (GSM), the same system used by AT&T and operated by most wireless carriers worldwide.

T-Mobile's network operates on different radio frequencies than AT&T's, which affects both the area of coverage for its cell phone towers and how well its signal penetrates buildings. As a result, T-Mobile customers often complain of network dropouts or lost signals, especially inside buildings. To correct this, in 2014 the company bought unused 700 MHz spectrum from rival Verizon and plans to begin to deploy it by the end of the year.

T-Mobile became the last of the "big four" U.S. carriers to get the iPhone, which it finally did in April, 2013. Since then T-Mobile has rolled out support for the iPad as well, even offering free limited bandwidth for iPads and other 4G LTE-capable tablets used by T-Mobile's postpaid customers.

T-Mobile's aggressive "Uncarrier" marketing efforts have branded the company as a rebel in the U.S. telecommunications market. T-Mobile's CEO John Legere carefully cultivates that image, using brash language on stage and occasionally staging publicity-enhancing antics like crashing AT&T's private party at CES 2014.

The bottom line, however, is that T-Mobile has not only grabbed headlines, but begun to significantly re-shape the way carriers do business in the U.S.