Geek.com demonstrates how easy it is to roll your own PBX in about an hour or two.
"Provided that the instructions are followed carefully, you too should be able to set up your very own switchboard/PBX system and all for the cost of the hardware of your choice. The beauty of Asterisk (Asterisk.org) is that it is free and offers all this functionality right out of the box. Asterisk is a software implementation of a hardware PBX and can run on a variety of hardware platforms."
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How to build and customize your own PBX with Asterisk
