VOIP rocks!
BT know this and they are planning on converting all their phone systems to VOIP (Voice Over IP).
Dont get left behind, download a VOIP softphone such as X-Lite now and try it out! :) Its fun :) You can use it to dial freephone numbers in america, japan, holland and supposedly the uk (though i havent managed to get this to work yet) Have sucessfully called Japan though :) There is also beta support for calling freephone French numbers.
In theory if you want to call a "normal" phone you can connect to a phone service provider/calling card provider on their freephone number then dial out from that to the number you require, for cheap calling rates. Though Im sure there are other ways of doing it.
You can get an easy to setup and use VOIP "softphone" called X-Lite, useable with the FreeWorldDialup (FWD) network here:
http://www.freeworlddialup.com/support/quick_start_guide