I was looking for a way to ring UK Landlines for free from my computer and found thishttp://www.voipcheap.com/en/index.html
it lets you do exactly that plus it lets you ring other countries for free like:
Hong Kong (+mobile)
United States (+mobile)
bad thing is you only get 300 minutes a week but thats still 5hours!
the software is free, and so far it seems good, ive had no problems so far (ive had it one day)