I found it fidlely to kick start the VOIP trial on my Linux desktop using xlite version2. MY PC is running OpenSuSe10.2. I can now however make calls successfully. If any one cares to help test my incoming calls my number is 01862808243 or sip 6748243.
I know this is not helpful - but I suceeded in trying out the VOIP in Linux (Ubuntu) on, if I remember correctly, "Ekiga softphone" (ubuntu application) and also the recommended download (xlite or similar). Both worked - Ekiga performed best but xlite had the better user interface. Ekiga has a "setup Druid" and asks you to prepay to ekiga.com (but that's optional). I gave up on both - I was using a dirt cheap ebay headset. The microphone was insensitive and the headphones needed to be turned up. To use it properly I would have to get a proper phone. I've just had a look in the ubuntu applications (apt-get etc) and twinklephone - see http://www.twinklephone.com/ looks rather good. I think it might only be available for latest version ubuntu. I might give it another go myself.