FreePBX vs Kerio Operator
FreePBX is a web-based open source GUI (graphical user interface) that controls and manages Asterisk (PBX), an open source communication server. FreePBX is licensed under the GNU General Public License (GPL), an open source license.
Kerio Operator is a small business phone system. Using VoIP, it simplifies voice communications while offering advanced phone features typically associated with enterprise phone systems like conference calls, call forwarding, call recording, ring groups, office voicemail to email, auto attendant, and call queues.