Telephone fraud cost UK businesses £750m in 1999, according to the latest figures from the Telecommunications UK Fraud Forum, an independent agency that tracks criminal activity across UK phone networks.
The most common form of telecoms fraud - illegally routing calls through a company's switchboard - has been known to cost an individual business up to £20,000 over a single weekend, with bank holidays being the peak period when companies are vulnerable to attack.
Vincent Blake, head of fraud control at Energis, the National Grid's telecoms spin-off, said: "It has been known for people to establish an empty shop in an inner-city area, where there is a high number of people wanting to make cheap calls.
"They can then hack into the switchboard of a medium or large company, anywhere that allows you to get an outside line, and sell each call for pure profit."
Mr Blake said that voicemail accounts, in particular, provided a window of opportunity for anyone intent on hacking in. "Each one is a portal to the switchboard. What the hackers are banking on is that at least one person in the company will have failed to change their access code from a simple default, such as 1111," he added.
According to Mr Blake, the hacker's job is becoming easier thanks to a raft of new websites that instruct people of how to access company switchboards and which, in some cases, sell hardware to assist criminals in carrying out the fraud.
Mr Blake said that while the majority of companies are aware of the threat of internet fraud, many fail to take precautions against telephone assaults.
Simple measures which can be taken to reduce the risk include deleting unused voicemails and checking bills for irregular call patterns.
Energis, which has launched a "best practice" guide on telecoms fraud control, also recommends taking out specialist insurance policies that provide cover in the event of an attack.
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