Jwb m8ty, to the best of my knowledge these days it is very difficult to use someones else's phone line/account to dial-up, unless you physically attach equipment to their line (which would be rather obvious??).
The most likely cause of your problem, is that at some point in your surfing (or when reading an e-mail) you've clicked a link and answered 'YES' to a question (which is not very clear), this then adds another dial-up networking connection to your m/c (or script to dial another server). Then when you click that link again, or goto the front page of an 'adult' web site and click enter, your m/c will drop the dial-up connection you started with (your default) and dial a premium rate (often international) phone number to connect to that providers server.
This has become the prefered method of getting money from people for many porn sites, they'll tell you 'NO CREDIT CARD REQ'D' etc. I have had several friends duped by this, one who managed to run up a £200 phone bill before BT advised him of the 'unusally high call charges he was being billed for'.
Sadly, this practice is very hard to stamp out, as the offenders are often in countries where your local telco regulator cannot touch them.
All I would say is complain to your telco and or any regulator you may have (here we have ICSTIS). Look carefully at your phone bill and see if you can track down the owners of the premium rate number/s you've been charged for. Also, check the dial-up networking connections on your PC, there should only be one for your isp (unless you have others that you know you have set-up). Also, look in add/remove programs for oddly named items you don't recognise. You could also ask your telco to bar the premium rate numbers, so you can't dial them.
Hope this helps
