Asterisk is an open source Personal Branch Exchange (PBX) system.
Like any PBX, it allows a number of attached telephones to make calls to one another, and to connect to other telephone services including the public switched telephone network (PSTN). A PBX also allows things like automated attendant, to creates messages that say "Press 1 to talk to a operator" and so on ;)
The O'Reilly book, Asterisk: The Future of Telephony will walk you through the process of configuring your first Asterisk system. From the book introduction page:
Many organizations are drawn to Internet telephony's promise of cost savings, and its ability to move data, images, and voice traffic over the same connection. Asterisk, an open source system than runs on Linux (and *BSD), offers the best option. This guide covers installing, configuring, and integrating Asterisk with existing phone systems, without the need for additional hardware.
Asterisk's flexibility comes at a price, however: it's not a simple system to learn, and the documentation is lacking. Asterisk: The Future of Telephony solves that problem by offering a complete roadmap for installing, configuring, and integrating Asterisk with existing phone systems. Our guide walks you through a basic dial plan step by step, and gives you enough working knowledge to set up a simple but complete system.
This book is published under the Creative Commons license.
Download Asterisk: The Future of Telephony PDF book (size 4.4M). Or try local mirror.