Rogue Diallers are pieces of software secretly placed on your computer by some of the nastier people who use the internet.

When you connect to the internet, these diallers quietly disconnect you and dial a premium-rate phone number instead.

So when you think you are connected to the internet as usual, you are in fact running up mammoth telephone bills on your account.

How do you get a rogue dialler?

Rogue diallers are bits of software placed on your machine, and are often hidden in other bits of software that you might download. They affect users of Microsoft Windows on dialup connections.

They are most commonly downloaded alongside software, music or pornography. They can also be installed on your computer through spam and pop-up windows.

Rogue diallers connect to premium-rate numbers

Some companies use diallers legally to bring in revenue from people who don't want to pay for things with their credit cards.

If you have broadband, rogue diallers cannot take over as long as your dialup modem is disconnected.

Piling on the pounds

Because they dial premium-rate phone numbers, you end up paying pounds per minute instead of pence. And you won't even know until months later - when you receive the phone bill. Your bill could be as much as £1500.

The way things stand now, if you contest such a charge on your phone bill, BT will give you six weeks grace in order to sort it out - before you have to pay. Unfortunately BT cannot investigate the problem themselves, but will refer the mater to ICSTIS, the premium-rate phone numbers watchdog.

This can take quite some time, and then it's entirely up to you to recover the costs. And this is particularly tricky as these companies often change their name and business address.