2013 has not been kind to AT&T (NYSE:T). Shares of the telecommunications giant have gained just 6% year to date while the Dow Jones (DJINDICES:^DJI) soared 22% higher. Worse, AT&T shares have been totally dead money in the last six months.

The Dow telecom duo started 2013 in high spirits. Heading into the April earnings season, both AT&T and Verizon (NYSE:VZ) were outperforming the Dow Jones by respectable margins. The seemingly unstoppable smartphone boom powered our largest wireless networks relentlessly higher.

T Total Return Price Chart

T total return price data by YCharts.

But when AT&T reported first-quarter results in April, the good times suddenly stopped rolling. It added a paltry 269,000 wireless customers in that quarter, and most of these eschewed high-margin package contracts in favor of AT&T's much less profitable Internet-only plans. Shares plunged 5% overnight and haven't approached April's multiyear highs ever since.

The smartphone rush took some hits over the summer. Apple and Samsung took turns showing signs of slowing smartphone growth, and both companies are looking forward to the next world-changing megatrend. Tablets haven't quite picked up the smartphone slack yet, Samsung's smart watch is a borderline-embarrassing launch, and Apple still hasn't even tipped its hand on its next big bet.

Meanwhile, smaller wireless operators are giving AT&T and Verizon a hard time. T-Mobile USA (NASDAQ:TMUS) split off from German mothership Deutsche Telekom, and is not afraid to challenge established industry traditions like contract lock-ins and feature-rich but pricey plan bundles. T-Mobile has started to steal customers from its bigger rivals, and the stock has absolutely crushed Verizon and AT&T during its short life.

TMUS Chart

TMUS data by YCharts.

Sprint (NYSE:S) sealed a game-changing $22 billion merger with Japanese peer Softbank and immediately used $1.5 billion to take full control of longtime network partner Clearwire. The new Sprint (now without the Nextel moniker) is capitalized to the hilt, armed with an impressive radio spectrum portfolio and controlled by a swashbuckling Japanese billionaire with his mind set on world domination.

These formerly unimportant rivals have become serious competition, and it shows in AT&T's disappointing 2013 share-price performance. Will 2014 be kinder? Stay tuned for our Foolish look into AT&T's prospects in the coming year.