Asterisk vs Kerio Operator

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Asterisk
An open source telephony switching and private branch exchange service for Linux. Information on downloading, installation of and support for the product. Asterisk is like a box of Legos for people who want to create communications applications. It includes all the building blocks needed to create a PBX, an IVR system, a conference bridge and virtually any other communications app you can imagine.
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Kerio Operator
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.