We recently reviewed Linphone 4.1.1 for Linux, an open source softphone developed by Belledonne Communications. We enjoyed the clean interface and simple conferencing functions, as well as the voice and video call quality.
Here's an excerpt from the review:
Linphone is an attractive SIP softphone choice for the typical user who just needs a phone for daily use. It has integrated messaging and call history for each contact, easily configurable audio settings, and a nice interface for audio conferencing. The voice and video quality were clear, although we found that video calling with the Linux client was incompatible with many hard phones.
There are a number of points that may be frustrating for power users and administrators on Linphone 4.1.1, such as being unable to prioritize codecs or configure the entire account in the GUI wizard. Many of these issues are relatively small changes, which makes them feel like a design flaw. OnSIP has raised several of these issues with the Linphone development team in the past and can only be hopeful that they will implement some of these changes in future versions.