I assume you're in UK, Boof. The answer is that you can't (unless someone pops up to contradict me who know better)
To a regular BT line, your computer's modem looks just like any other telephone and the exchange thinks you are busy.
I believe that if you have a "call waiting" service available where you live, the "beeps" that puts on the line are likely to drop the modem connection, but I don't think it's guaranteed.
Your best answer would be to get another line - and I believe my son got a deal where his calls to his ISP are free (which is why I often see him logged on all night while he downloads games as he sleeps!)