I used to use a thing called BTNetchat - it allowed incoming calls to be answered through the computer for a small fee. It never worked well with poor delays throught the PC and didn't like firewalls etc.
As BRL says, I now use call diversion (one of the BT select services). You can select to divert calls when unanswered, when busy or all calls. I set it to divert to my mobile when I am online - any incoming calls show the number on the mobile - you can answer or ignore it. The only snag comes when I leave it set up when I'm out, try to ring home when my son is online, and get diverted back to myself - of course my mobile is busy and I end up speaking to my own voice mail!