Please note that I did this on my Ubuntu 10.10 machine.
Here we go:
1) Download and install Linphone from http://www.linphone.org/ or you can simply use Ubuntu Software center.
2) Open Linphone. You can find this under, Applications --> Internet --> Linphone
3) Then select, preferences from Linphone menu. Go to "Manage SIP Accounts". Click Add button and enter following details.
- Your SIP identity: sip:<smart_voip_userName>@smartvoip.com. eg: yourName@smartvoip.com
- SIP Proxy address: sip:sip.smartvoip.com
- Keep other settings as is and click 'OK'
That's it! We are done!
Enjoy your calls to loved or non-loved once! :-)