On my latest telephone account I noticed frequent calls, mainly in the middle of the night, to United Kingdom Number) 0870 6080237. I have dialled the number, and I can hear data tones, presumably indicating that I am coneected to a computer of some sort. This has been happeningwithout my knowledge for some time. It appears to me that my computer has been compromised. I have a broadband internet connection, and have alsways been running zonalabs anti virus software ever since I purchased this computer, and as far as I am aware it is configured correctly.Can anyone advise me what I should do? How can I stop this happening, and how can I prevent it from happening again?

Operating System:Windows XP Pro
Product Name:ZoneAlarm Internet Security Suite
Software Version:6.0