Typically, hackers armed with a personal computer and software downloaded from the internet gain access to a company's private branch exchange, then make calls at the PBX owners' expense.
Amateur hackers do this for their own benefit; professional hackers "sell" the stolen line capacity for gain.
"This type of telephone fraud can cost business dearly if not detected early," said Vincent Blake, Energis' fraud control manager.
"Small businesses are particularly vulnerable since they tend not to have the budget for advanced security systems or a dedicated telecoms manager," he added.
Publishing the results of its study, Energis is seeking to explode the myth that telephone fraudsters hack into large companies on the grounds that size gives them anonymity. The company also hopes to market its via- a-telephone fraud helpline.
Energis is stepping up its marketing efforts as part of plans to sell off a majority of its shares via public flotation and bond issue.
On 22 January, National Grid announced that it would sell 60 million ordinary shares and pounds 250m of bonds, exchange- able for 14.7 million shares in Energis, and so reduce its stake in the company from 74 per cent to 49.5 per cent.
National Grid is cutting its stake in Energis to capitalise on the company's high value and enable it to enter the FT-SE All Share index.
HSBC Investment Bank is managing the sale of the shares and bonds, which is due to be culminated this week.
The sale was timed to take advantage of investor interest in telecoms, internet and multimedia companies. In August, Energis bought Planet Online, the company that carries traffic for Freeserve, the UK's number one internet service provider.
Analysts said investor interest in the Energis share sale has been high. Energis shares have risen sixfold since they were first sold to the public in December 1997.
Once quoted, Energis is likely to enter the benchmark FT-SE 100 index of the hundred biggest companies, because its market value is already larger than that of some members of the index.
Entry into the FT-SE should also further boost demand for the company's shares, because membership of the Footsie 100 will require investors who track indexes to buy the stock.