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AT&T Inc. detailed its plans for beefing up its often-maligned network Thursday, one day after being tapped to carry another closely watched Apple Inc. device that will likely add even more traffic to its airwaves.

The carrier's attention to network issues reflects its deep reliance on the iPhone for growth. AT&T reported that it activated 3.1 million iPhones in the fourth quarter, the second-highest total since the popular phone's 2007 launch, a surge that helped push spending on data plans and overall wireless revenue higher.

Apple gave AT&T an important vote of confidence this week by making it the carrier for the new iPad, but the consumer-electronics company made clear that it is monitoring AT&T's plans for improving its network.

Complaints about AT&T have centered on service in New York and San Francisco, where people live in tight quarters and tend to be heavy users of mobile devices like the iPhone. Thursday, John Stankey, chief executive of AT&T's operations division, said the company has spent months putting in new radio boxes in those cities and plans to build more cell towers in high-density areas like transportation hubs.

"We expect improvements in both markets in the coming months," Mr. Stankey said. In Manhattan, in particular, capacity will increase by a third by the end of the quarter, he added.

AT&T plans to spend $18 billion to $19 billion on its networks this year, including $2 billion more on the wireless network than last year.

Mr. Stankey said the carrier plans to add twice as much wireless capacity as it did last year and connect three times as many towers by high-speed fiber cable. The total expenditure is more than some analysts had expected, and it follows deep cuts in capital spending last year.

While it isn't clear how much the spending will improve the network or consumers' perception of it, the moves show the challenges posed by AT&T's ties with Apple. The iPhone powers the carrier's wireless growth, but it remains a major cause of the data congestion plaguing parts of AT&T's network.

AT&T added 2.7 million net subscribers in the fourth quarter, but only 910,000 of those fell into the more-ucrative pool of customers on service contracts. Meanwhile, iPhone activations brought more than 1 million new contract customers to AT&T.

The strong iPhone sales helped quarterly wireless revenue rise 9.2% from a year earlier to $12.6 billion, even as revenue from AT&T's fixed-line business fell 5.3% to $16.2 billion.

Overall, AT&T posted a profit for the quarter of $3 billion, or 51 cents a share, up 26% from a year earlier. Revenue fell marginally to $30.9 billion.

AT&T is offering two iPad service plans, neither of which requires the usual contract. Some have wondered whether the unlimited plan -- priced at $30 a month -- could be too cheap for AT&T to make money, but company executives were upbeat.

"The financial characteristics are attractive," said AT&T Chief Financial Officer Rick Lindner. The deal doesn't give AT&T much leverage over subscribers, but it leaves customer service and billing to Apple. "It is a substantially different model" from other service contracts, he said.