The firm is also selling wholesale capacity to other operators.
The firm had a fibre optic backbone of 8,000 kilometres and a further 4,000 kilometres would be added in 2011.
Chief executive Greg Young said declining depreciation was indicating that its network was ageing, signaling need to "accelerate modernization" to meet "explosive data growth and broadband demand."
SLT is state joint venture with Malaysia's UT group.
In 2009, amid a down turn in the industry investments had been curtailed. In 2010 capital investments had been increased to 4.9 billion rupees but higher levels were planned for 2011.
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SLT is changing over to a 'next generation network' which can handle both data and voice through packet based transmission.
"Already almost 20 percent of our customers are served by this technology, with plans to increase this to over 40 percent by end 2011," Young told shareholders in the firm's annual report.
The move will cut costs and improve quality and reliability of t