If you are looking for a free, multi-platform, multi-modal, contact-centric SIP client with a solid SIP stack and refined user interface--look no further than Blink.
Blink also does conference calls, multiple simultaneous conferences, excellent multiple call handing, HD voice and Google contacts integration. Blink has a features call "bonjour" that allows people on the same network to communicate without a SIP server involved! (simple in house survivability?) Oh, I almost forgot that is has client call recording. To top everything off Blink has a very refined UI and even the sounds are very elegant and provide excellent feedback.
Here is my quick look at the beta of Blink for Windows:
There have also been hints by a Blink insider that there may be a mobile edition on the roadmap. No promises there.
The Blink SIP client combined with snom ONE small business phone system can make an entirely free and extremely high quality phone system. AG Projects (maker of Blink) will also host the back end SIP server for you.
Watch for the soon coming release of Blink for Windows:
Twitter Blink for Windows Announcement: