By Thomas Gryta 

AT&T Inc. added 854,000 of the wireless industry's core customers in the last three months of the year, as profits and customer retention showed signs of strain from the heightened competition among the nation's biggest carriers.

The fourth quarter, usually the busiest and least profitable for wireless companies because of the costs related to phone upgrades, included strong demand for Apple Inc.'s latest iPhone at a time when the carrier used promotional deals to battle with rivals for customers.

With little room for growth in the U.S., where the market is saturated, AT&T is spreading into new businesses through the acquisition of satellite television provider DirecTV and deals to acquire two wireless companies in Mexico. AT&T expects all the deals to close by midyear.

"We will exit 2015 a very different company," Chief Executive Randall Stephenson said on a conference call.

The company is also coming to the end of two years of intense investment to upgrade its wireless and wired networks. The Dallas telecom giant reiterated that it would spend around $18 billion in 2015, down from about $21 billion in 2014.

Amid all the strategic moves, the company's net income was hit by an already disclosed $7.9 billion charge for pension-related costs. For the three months ended Dec. 31, AT&T reported a loss of $3.98 billion, compared with a profit of $6.91 billion a year earlier. Revenue rose 3.8% to $34.4 billion.

The level of monthly service cancellations--called churn in the industry--ticked higher to 1.22% in the fourth quarter from 1.11% a year earlier, as promotional deals from competitors prompted customers to jump to deals at rival carriers.

The company warned last month that the cost of promotions would also hurt the carrier's wireless-service margins, which fell to 34.8% from 37.4% a year ago.

The wireless industry is under pressure from all sides. T-Mobile US Inc. and Sprint Corp. have been working to take customers from top carriers AT&T and Verizon Communications Inc. Meanwhile, Google Inc. is working on offering a wireless service and Cablevision Systems Corp. said Monday it would offer service through its network of Wi-Fi hot spots.

AT&T has made its own moves, including moving much of its customer base to cheaper plans.

This year, AT&T forecast expanding margins, more cash to cover payment of its dividend and per-share earnings growth in the low single-digit range. The company plans to give a more detailed forecast after the DirecTV deal closes, which AT&T expects to happen in the first half.

AT&T is also involved in the Federal Communications Commission's $45 billion auction of wireless airwaves and investors are itching to know how much it is spending. The company declined to comment, citing the auction's rules, but New Street Research estimates that AT&T may spend $20 billion to $22 billion.

Meanwhile, AT&T is also moving into Mexico. Since last reporting earnings, the company is now the country's No. 3 wireless carrier through its $2.5 billion deal to buy Grupo Iusacell SA, and Monday it announced plans to buy Nextel Mexico from bankrupt NII Holdings Inc. for $1.88 billion.

A big question is whether AT&T will try to gain more scale in buying Mexican wireless assets to be sold by former business partner Carlos Slim's America Movil.

Mr. Stephenson echoed previous comments from the company about the uncertainty around the assets that might be sold. He stressed that AT&T believes it has what it needs to succeed in Mexico "whether America Movil sells or not."

"We have all we can handle right now in Mexico," he said.

Write to Thomas Gryta at thomas.gryta@wsj.com

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