For Bria phones, if you receive a "500 Server Error" upon startup, it could be a MAC firewall issue.
On your Mac go to "System Preferences" and then choose "Security and Privacy". Choose "Firewall" and disable the Firewall (turn it off.)
Restart your Mac and then bring up Bria.
The issue is that the packet OnSIP receives is malformed in that it's missing the final "end of packet" character on the REGISTER. We then return:
SIP/2.0 500 Server error occurred
Because we could not read the packet. Turning off the MAC firewall seems to resolve the issue.