Asterisk® is self-described as "the world's most powerful and popular telephony development tool-kit." It's not a phone system so much as it is a set of building blocks that allows you to construct custom phone-based applications. From the project web site among the various building blocks Asterisk offers are:

• Drivers for various VoIP protocols.
• Drivers for PSTN interface cards and devices.
• Routing and call handling for incoming calls.
• Outbound call generation and routing.
• Media management functions (record, play, generate tone, etc.).
• Call detail recording for accounting and billing.
• Transcoding (conversion from one media format to another).
• Protocol conversion (conversion from one protocol to another).
• Database integration for accessing information on relational databases.
• Web services integration for accessing data using standard internet protocols.
• LDAP integration for accessing corporate directory systems.
• Single and mult-party call bridging.
• Call recording and monitoring functions.
• Integrated "Dialplan" scripting language for call processing.
• External call management in any programming or scripting language through Asterisk Gateway Interface (AGI)
• Event notification and CTI integration via the Asterisk Manager Interface (AMI).
• Speech synthesis (aka "text-to-speech") in various languages and dialects using third party engines.
• Speech recognition in various languages using third party recognition engines.

These functions can be layered on top of one another to construct intelligent telephone-based applications. With over two million current users Asterisk is recognized as the world's most popular open source telephony project.

The JumpBox for Asterisk provides the simplest path for getting started with this tool kit. It bundles the popular FreePBX® GUI for administrating Asterisk and comes pre-installed with the Sipstation plugin to make it dead simple to get started. For a quick primer on how to set it up to make your first call to a real telephone check out the screencast below.