(Adds details on 3G capex)
* Q4 net profit up 131 pct on year, service rev up 9 pct
* Raise 3-year investment budget to 70 bln baht
* Shares extend earlier gains after earnings announcement
BANGKOK, Feb 7 (Reuters) - Advanced Info Service Pcl (AIS) , Thailand's top mobile phone operator, reported a surge in quarterly net profit, boosted by strong voice and data services and higher handset sales after the launch of Apple's iPhone 5.
The firm raised its three-year investment budget by 40 percent to 70 billion baht ($2.35 billion) to speed up 3G network installation as it aims to tap strong domestic demand in the fast-growing country.
The company is aiming for 2013 service revenue growth of 6-8 percent after rising 11 percent in 2012, it said in a statement on Thursday.
AIS, 21 percent owned by Singapore Telecommunications Ltd posted net profit of 8.5 billion baht ($284 million) for October-December, up 131 percent from a year earlier when it booked 3.2 billion baht in expenses for network impairment.
The result was marginally lower than the average forecast of 9 billion baht from 13 analysts polled by Reuters.
Excluding the impairment costs, its profit from normal operations rose 23 percent from a year earlier. For the whole of 2012, net profit rose 57 percent to 34.9 billion baht.
The country's third-most valuable company has about 50 percent of the cellular market in Thailand, ahead of Total Access Communication's roughly 30 percent.
At the end of December, AIS had 35.7 million subscribers after adding a net 419,800 in October-December.
AIS had previously planned to spend 50 billion baht in three years on 3G expansion.
Shares in AIS, valued at $21 billion, have underperformed the market so far this year due to concerns about a possible drop in dividend payout as a result of higher investment.
After the earnings announcement, AIS shares extended earlier gains and rose 1.5 percent, while the broad index was 0.5 percent lower.
The company said it intends to pay dividend at 100 percent on consolidated earnings this year. ($1 = 29.76 Baht) (Reporting by Khettiya Jittapong; Editing by Daniel Magnowski)