TOKYO (Reuters) - Japan's Softbank Corp is considering offering free calls between its mobile phone and Web-based fixed line phone users to boost its subscriber base, sources close to the matter said on Tuesday.

Softbank has already been offering a similar service on a trial basis in three prefectures in Japan including the northern island of Hokkaido, but nothing has been decided on making it a nationwide service, a company spokesman said.

Softbank is Japan's third-largest cellphone operator behind NTT DoCoMo Inc and KDDI Corp . It also offers Internet Protocol (IP) phones as part of its high-speed Internet access service.

The Nikkei business daily said in its Tuesday morning edition the free cellphone-IP phone calls may lower Softbank's per-user revenue, but that the company aims to boost total sales by attracting more customers.

Softbank, which bought Vodafone's Japan unit in 2006, has outpaced its competitors in netting new users with low-cost price plans and a marketing blitz.

Shares in Softbank were unchanged at 2,070 yen by midday, underperforming the Nikkei average , which rose 0.5 percent.

(Reporting by Noriyuki Hirata and Kiyoshi Takenaka)
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