Our latest phone review considers Jitsi Desktop for Ubuntu, the premier open source softphone for Linux. Although the phone is free, it has a robust feature set similar to many commercial softphones. Here's an excerpt from the review:
Given that the landscape for native Linux SIP clients is fairly bleak, Jitsi stands out as a very useful piece of software. We use it heavily at the OnSIP offices because of its robust configuration, easy customization, and in-depth troubleshooting functions. Although it’s not as sleek in appearance many commercial soft phones, its rich feature set and easy availability make up for the lack of aesthetic appeal.
The Jitsi project is an exciting combination of servers and clients used for video bridging and unified communications, so it's not surprising to see that the strongest aspects of Jitsi Desktop were in conferencing and video calling. Although there are definitely aspects of the phone that were head-scratching for a new user, with a little education and guidance, Jitsi Desktop can become a very reasonable go-to phone for every day usage.