Modem problems can occur when you download (sometimes unwittingly) software known as a 'dialler' that alters the number your computer uses to connect to the Internet. These diallers connect to the internet via a premium-rate (090) or international (00) telephone number rather than the number supplied by your ISP.
Whilst there are some 4,000 diallers registered with ICSTIS, the UK premium rate industry regulator, working perfectly legitimately, some websites will dupe you into producing high call charges through downloading a supposedly 'harmless' program, application, or service that will include an unregistered dialler. The dialler software is often invisible to users, running discretely in the background so that you do not notice any disruptions. Unexpectedly high call charges associated with unregistered diallers are often only discovered when you receive your phone bill.
Full Overview Plus Software @ BT's Modem Protection Website - Requires Form To Be Filled In.