http://www.euteliavoip.com/
If you want to call with your PC
you just need to download a free easy to use software (Phone Software) and then configure it with the correct parameters. If you need help, please see the manuals.
If you want to use a VOIP or traditional telephone
(also portable), you need to connect to a Voip Gateway. You can buy Voip telephones and Gateways on EuteliaVoip Shop.