This is a free to use SIP softphone running directly from your java enabled web browser.
Feel free to connect to any VoIP server and call any phone number via any VoIP service provider.

If you don't have Java installed you can install from here.
You must allow this applet to access your computer (Accept the certificate, Click on "Allow blocked content" in IE).
Otherwise it cannot access your microphone and cannot initiate phone calls.
If Java doesn't work for you, than click here to download a free desktop softphone.