It's a common sight yet it is surprising: many people emerging from underground metro stations checking their mobile phones for missed calls and messages. Common, because so many people have mobile phones, and surprising that there is no network signal in the underground.

In a business where new operators are constantly striving to offer better services than incumbents, remarkably few of the world's cellular operators have targeted the large and captive audience underground. Perhaps many passengers are grateful to be spared the sounds of ringing phones and half conversations (the tinny beat of Walkmans is bad enough), but operators' promises of 100 per cent coverage in cities ring somewhat hollow when millions of people daily are out of range while commuting to and from work.

Not so in Hong Kong and Singapore. In 1993, Hong Kong Telecom became the world's first operator to offer complete GSM coverage in a metro system. The underground cellular coverage coincided with the launch of its GSM network, and was intended to attract new customers in a competitive environment where customers complained if calls were dropped in elevators or road tunnels. The existing analogue networks were being phased out, and there were three GSM operators starting up.

According to the then commercial and business manager of the project, Mr Richard Midgett, the approximately $20 million (#17.8 million), 18-month underground project paid off, and the company was rewarded with a significant market advantage over competitors. In the race for customers it went from second place to first.

Mr Midgett, now general manager of international business development and chairman of the GSM Association, said the project gave Hong Kong Telecom a "major competitive advantage", fulfilling its objective of allowing a user to enter one of Hong Kong's 38 metro stations, descend the escalators, board a train, and emerge from another station while maintaining a mobile call throughout.

He added that another advantage of the underground mobile network was that it lessened the load on the above-ground network. Surveys had shown commuters were using alternative transport in order to be able to stay within mobile coverage, he said, and furthermore, because the radio signals in the 43 kilometres of tunnels did not interfere with those above ground, the same scarce frequencies could be used above and below ground.

But covering underground rail networks is expensive. Not only are the service costs high, as installation and repair work has to be carried out at night when the trains are not running, but the technological challenges are greater. Mr Patrick Scodeller, who, like Mr Midgett, formerly worked for Cable and Wireless, was in charge of the technical aspects of the project.

He said the stations were covered with ordinary antennas, while the tunnels were lined with leaky cables, so called because the shielding is deliberately holed to allow the signal to leak out.

But within a year came proof that the investment was justified: one of the other two operators, SmarTone, leased capacity in the system. Then, in 1995, the Mass Transit Railway (MTR) Corporation bought the system from Hong Kong Telecom, for what Mr Midgett described as a "modest consideration" above the development cost, and the system was upgraded to allow the third operator, Hutchison, to gain access.

Now the MTR effectively provides a sophisticated antenna system, while the operators are responsible for providing the network equipment up to the so-called base stations, the last equipment before the antenna.

Mr Scodeller now works for another Cable and Wireless operator, Mobile-One in Singapore. Coincidentally, Singapore's metro system is the only other one in the world with complete digital cellular coverage. In this case, said Mr Scodeller, the project was less motivated by business than by the government's desire to ensure total coverage. Unlike in Hong Kong, the Singapore government owned both the phone company (Singapore Telecom) which installed the network and the metro company, so negotiations were simpler.

Paradoxically, the commercial success of the Hong Kong Telecom project seems to have delayed other operators. In Britain, the main cellular operators have been talking to London Underground, but wrangles over fair access for all have delayed progress for years, according to one analyst.

The reasons are not technical; nor are they all related to costs. According to Ms Giulia Rancati, a European mobile analyst with the Massachusetts-based research firm International Data Corporation (IDC), the delays have been more a legal than a technological issue, adding that the operators have told her they are still "miles away from a legal solution".

"They weren't even saying they were in the process of a solution," she added. The only part of London's network she was aware had mobile coverage was directly under Heathrow airport.

According to Chicago-based Andrew Corporation, a prominent contractor in the deployment of radio networks, there are few other networks being planned. Mr Matt Melester, business area manager for distributed communications systems, described underground mobile networks as something "about to happen", and described Asia was most dominant, with Europe second, followed by the US.

Mr Melester said there were small networks in Madrid, in Store Baelt in Denmark, and in some German cities, and there were tenders for business in New York and Boston. Eurotunnel has been approached, he said, but he stressed this was still at a very early stage.

Mobile-phobes can be thankful that most of the world's underground networks look set to remain mobile-free zones for some time yet. Mobile users, meanwhile, are reminded to check for missed calls when resurfacing.

Eoin Licken may be reached at elicken@irish-times.ie.