The open source Twinkle softphone has released a new version today. Version 1.4.2. This version is seamlessly integrated with the Diamondcard service. Available for Linux only.
You can make calls, receive incoming calls, and also send SMS's through Twinkle. Multi-line and conferencing too!
For more information go to the Twinkle phone web site.
For help in using Twinkle with Diamondcard go to the Twinkle phone page.