Choosing a phone for any VoIP system is difficult if it is important to you that more advanced features work as well. Every SIP phone is capable of initiating and receiving calls. More advanced features require phone firmware to be compatible with the SIP standard and implement all the necessary features.
sipXecs is the only open source IP PBX system that auto-configures all the phones. No more logging into phone Web server and configure one phone at a time. sipXecs provides plug & play. This not only includes basic phone configuration, but also includes advanced features such as directory, speed dial, BLF, firmware management, phone auto-discovery, and automatic remote restarts.
From a technical perspective the best phones are those that have been thoroughly tested and the phone manufacturer has been willing to fix issues that came up during such testing
Just because configuration support exists in sipXecs does not indicate that all system features will work with that phone.