BT have recently started BT NetChat. If you have a BT line, irrespective of ISP, you can receive incoming calls whilst online - and answer them too through your PC. You can make outgoing calls to other NetChat users.
Or so it says - I'm not connected yet.
Look at:
http://www.btnetchat.com/btnc/BTNetChat.htm
I think freeserve do something too.