Dependant upon which country / ies you want to dial for free would dictate which particular version you download.
But assuming Voipstunt covers your needs, you download the software from the homepage
www.voipstunt.com Set up your account and credit it with €11.60 . Of which only €10 is credit, the rest is V.A.T.
Once you open the program there is a box labelled "Direct Callback Nr". In here you type your number. At the bottom of the screen is a blank box for you to enter the target number.
Press the green dial button.
Your phone will ring, which you then pick up. You will hear a ringing tone as it dials the target.
Dependant upon the origin and destination would dictate your final charge.
Eg with voipstunt the callback connection fee is €0.03 and the cost to dial England is free. So from your UK landline to another UK landline, the total charge would be €0.03 for a max of 60mins, with a total of 300 free mins in a ROLLING 7 DAY PERIOD not a week.
But if you wanted to connect Afganistan with Angola, the the call would be €0.03 connection + per min charges of €0.53 (€0.29 Afg + €0.24 Ang)
This way no equipment is necessary other than an internet connect to initiate the call.
Hope this helps.