*If you want to call within Qatar, the Qtel Telephone is free of charge after paying a rental.

*If you want to make an international PC to PC call then you may use Skype:
http://www.skype.com/intl/en/get-skype/

*If you want, PC to Phone/mobile phone call, then you may use Voipwise...
The link says further details/charges:
http://www.voipwise.com/en/calling-rates.html#lett
Or

http://www.voipwise.com/en/calling-rates.html