The telecoms giant BT has blocked 1,000 numbers allegedly associated with rogue internet diallers.
People affected by scam are landed with huge phone bills
It is part of a drive against scams which inadvertently download software and then dials a premium rate number when users log on.
Victims in the UK have been landed with huge telephone bills.
BT is trying to minimise the number of victims of the scam and has sent e-mails to all its dial-up customers, offering advice on how to avoid it.
It is a big problem with around 45,000 cases already dealt with by the company, and a further 9,500 awaiting resolution.
One of the options is to have all premium rate numbers barred. This is offered free by BT. A removable bar for premium rate and international calls is also available for £1.75 a month.
BT is sending advice to its customers on how to protect PCs with all BT bills.
INTERNET DIALLER SCAM
Only dial-up internet users will be affected
Bills up to £1,500 are not uncommon
Companies lease numbers from one of around 70 network operators
The 1,000 blocked numbers are mainly international ones
Nearly 10,000 cases are being investigated
Most people will end up having to pay out
BT estimates about 70% of cases are simply due to customers not understanding charging mechanism
An e-mail outlining the dangers and how to avoid them will also be sent to all narrowband customers in the next few weeks.
The scam relies on people having a dial-up internet connection and will not affect those with broadband, as long as their dial-up modem is disconnected.
In some cases the diallers are installed on people's PCs by programs hidden in spam e-mail or web pop-ups.
But in many cases the problem lies with customers who have simply not read or understood the terms and conditions of diallers they install to download software, music or pornography.
Diallers are commonly used by gambling and pornography sites as a way of charging people, without the need for a credit card.
"BT is doing everything in its power to stop this menace," said BT Group Managing Director Gavin Patterson.
Anyone questioning an unusually high bill will have the payment suspended by BT for six weeks.
However, BT itself cannot investigate the matter and the details are forwarded to the premium rate numbers watchdog Icstis (Independent Committee for the Supervision of Telephone Information Services).
According to BT, it can take months for an investigation to be completed.
At the end, the customer in dispute will be sent the details of the company that has charged it and can pursue their case directly with the firm.
However, many of the companies operating in this area are hard to chase up, changing telephone numbers and addresses regularly.
BT says few people have got recompense from such firms.
It admits that the number of people operating in this manner means its measures are only "stemming the tide".
It wants the industry to do more and is calling for the operators who offer companies premium rate numbers in the first place to take a closer look at who they are doing business with.
It is an issue that Icstis is also looking at.
In an effort to distance itself from the scammers, BT has foregone its share of the profits from such calls.
Its revenue share, around three pence of the £1.50 a minute charge, will instead be donated to the charity ChildLine.