Sir.-Many readers will be aware that when using the internet there is a risk of incurring expensive phone call charges by inadvertently downloading a piece of software known as a "dialler" onto a computer.

Most diallers are perfectly legitimate. But others can effectively "hijack" a computer and dial premium rate or international numbers without the user's knowledge.

Tens of thousands of dial-up internet users have fallen victim this year.

BT is doing everything it can to tackle this menace. In the past three months we've blocked 1,000 UK and international numbers associated with this problem. We're also e-mailing all our internet customers to warn them and give them advice.

Customers can ask BT to bar, free of charge, all premium rate calls - 50,000 customers have taken advantage of this since July. There is also a selective barring service, including international calls, which costs £1.75 a month.

An explanation of the methods employed to entice customers to download these "rogue diallers" and advice on protection is available at www.bt.com/premiumrates

The battle is not just BT's. Companies who use rogue diallers lease the telephone numbers from about 70 telecoms operators. BT is legally obliged to pay the operators for calls which our customers make to their numbers. They, in turn, pay the companies, including those who are responsible for the rogue dialling problems.

BT is calling on these other operators to take more responsibility for the actions of the companies with whom they do business.

The regulating body, the Independent Committee for the Supervision of Standards of Telephone Information Services (ICSTIS), is working with the industry to solve this problem. The ICSTIS website at www.icstis.org.uk provides details.

BT is making no profit from this problem. From every £1.50 of a premium rate call, BT retains only 3p. We are donating all of this share to the children's charity Childline.

-Patricia Vaz, BT South East regional director, e-mail.