Media playback is unsupported on your device Media caption How the Softbank share sale could change your life

The mobile phone unit of Japanese tech giant Softbank had a disappointing debut on the Tokyo stock market.

The firm raised as much as 2.6 trillion yen ($23bn; £18bn) by selling shares at 1,500 yen each in one of the world's largest ever stock offerings.

But by the close of trade in Tokyo, shares were down 14.5% from the price set for its initial public offering.

Analysts said the disappointing debut was not entirely unexpected.

"Softbank wasn't as popular an [initial public offering] as the market had expected," Singapore-based market expert David Kuo told the BBC.

"It was oversubscribed, but not as much as hoped."

Price war

Investors may have been wary over a number of developments in Softbank's business.

There is the prospect of a price war after Japan's two biggest mobile phone operators said they would cut the cost of calls next year.

The firm's reputation suffered when its network failed in large parts of Japan two weeks ago.

Also, Softbank may have to replace expensive kit made by Huawei, the controversial Chinese firm, after the Japanese government said Huawei equipment threatened national security.

Originally a telecoms firm, Softbank has become a vast conglomerate covering robotics, chips and investments.

The firm was founded by Japan's richest man, Masayoshi Son.

What does this sale mean for Softbank?

While you could already buy shares in Softbank itself, its telecommunications unit, called Softbank Corp, is now also available for public purchase.

The firm's initial public offering (IPO) - a way for a company to raise new capital - was seen as cementing Softbank's move from a domestic telecommunication provider to a global tech investor.

It was expected to be one of the biggest offerings of shares on any stock market to date.

Chinese e-commerce giant Alibaba listed for a record $25bn in 2014. In comparison, Facebook raised $16bn when it went public in 2012.

Mr Kuo, who is chief executive of the Motley Fool in Singapore, said however that the stock price should pick up once investors understood better how to value Softbank's telecoms arm.

"As an entrepreneur, Masayoshi Son had the freedom to invest as he wished," Mr Kuo said.

"But the [newly listed arm] now has to play by the harsh rules of the market. It will be valued on revenues, profit and cash flow."

What is Softbank?

Softbank started as a telecoms company, but has spread out through many deals and investments.

The firm has moved into robotics, bought UK chip firm ARM Holdings and invested in satellite start-up OneWeb and ride-sharing firms, as well as in self-driving technology with Toyota.

Image copyright EPA Image caption Softbank is behind the robot Pepper

It also acquired Vodafone's Japanese operations and US telecoms company Sprint.

The firm set up a venture fund with Saudi Arabia, which focuses on emerging technology and has about $90bn at its disposal.

Saudia Arabia is the fund's major investor. Other backers include Apple and Foxconn.

But Softbank has come under criticism for co-operation in the light of the recent killing of Saudi dissident journalist Jamal Khashoggi.

Company chief Masayoshi Son condemned Khashoggi's murder, but said Softbank must continue to work with Riyadh.

Who is Masayoshi Son?

Mr Son founded Softbank and has guided it to become one of the world's biggest technology companies.

Image copyright Getty Images Image caption Masayoshi Son is Japan's richest man, according to Forbes

The entrepreneur - Japan's richest man, according to Forbes Magazine - is known for having an eye for firms with big potential and for spotting transformative industries and trends.

He saw the potential in e-commerce before many others and was an early investor in Alibaba.

Often described as the Steve Jobs or Bill Gates of the Japanese business world, he's seen as someone quite different from the country's more conservative corporate culture.