OK if we are sticking with Linux, I used Skype for quite a while with the Skype software for Linux. I mainly used that because it was written for Linux and worked very well. I never tried it with other apps as a plugin so I couldn't really comment on that score. No problems with security although I had to port forward to the computer I was using it from. The hardest part was getting the usb phone to ring for an incoming call. I managed it in the end but I can't remember how. Video of course slows things down and is not quite mature enough in my opinion, but you always have the option of not transmitting video. The audio was quite good as was the ability to speak to friends free of charge. The landline calls were very cheap as well. I don't use it now as I have lost a number of friends from my circle (that's another story) and it's as easy to email and instant message. Have a go, you will enjoy the experience.
