AIS earnings fared better than regional peers. Telstra, Australia's largest phone firm saw first-half profit fall 36 percent on heavy spending to win back customers from rival Singapore Telecommunications.

SingTel, Southeast Asia's biggest telecoms firm, posted a 2.2 percent fall in October-December underlying net profit, dragged down by its Indian associate Bharti Airtel, which is under pressure from a scandal involving the cheap issuance of licences and mobile spectrum in 2007/08.

In Thailand, the telecoms sector is suffering from regulatory uncertainty resulting from the government's investigation into operating concession contracts made under past governments.

AIS shares were hardest hit, falling 17 percent in last three months on concern that the company would seek substantive compensation for losses from the changes in concession.

Before the earnings announcement, AIS shares were up 0.3 percent at the midday break, while the main index was down 0.8 percent.

AIS, 21 percent owned by SingTel, posted an October-December net profit of 5.8 billion baht ($188 million), up from 4.1 billion baht a year earlier.

With a slower pace of economic growth, the market leader forecasts service revenue to rise 4 percent this year when voice revenue is expected to be flat as competition remains benign, it said in a statement.

Revenue from data is expected to rise 25-30 percent this year, helped by the popularity of low-end smart phones and social networking, the company said.

"Data will be key driver for the company's earning growth over the next few years," said DBS Vickers Securities analyst Charasit Vuttigrai.

In Thailand, where mobile penetration rate passed 100 percent of the population, revenue from data usage accounted for 17 percent of AIS's revenue, versus an average 25 percent in Asia.

AIS, Thailand's seventh-biggest listed company with a market value of $7.7 billion, has about 50 percent of the mobile phone market, ahead of Total Access Communication's which has stake of roughly 30 percent.

Earlier this week, TAC reported a better-than-expected 37 percent rise in quarterly earnings.

Fourth-quarter average revenue per user (ARPU) from prepaid subscribers rose 1.5 percent from a year earlier, while ARPU from post-paid clients increased 2.1 percent due to growth from non-voice services, AIS said.

At the end of 2010, it had 31.2 million subscribers after adding a net 2.4 million last year. That compared with 2 million net additions for TAC, which had 21.6 million users.