You can certianly use your Windows dialler to phone out, using your normal BT/Virgin line, but I think you will need an ordinary modem - don't think your broadband modem will work, but you can always give it a try. Same principle as for Faxing.
I used to be able to do this on one of my old Windows 98 machines (currently stuck in a corner not being used). You certainly need a normal dial-up modem connected to the standard telephone line. It is then like making any telephone call and shouldn't affect broadband speed. I assume that your telephone calls are made with Virgin and that is not just your ISP. However incoming calls can be a pain and I have found it much more convenient to have a standard phone on the desk beside the PC.
Why not sign up to Skype, you can talk to people all over the world, land line or mobile, for not a lot. If you are using skype to skype "like wot i does" to my daughter who lives and works in spain it costs me NOTHING..