Asterisk is a complete PBX in software. It runs on Linux, BSD, Windows and OS X and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in four protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.
Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing. It has support for three-way calling, caller ID services, ADSI, SIP, H.323 (as both client and gateway), MGCP (call manager only) and SCCP/Skinny (limited). Check the Features section for a more complete list.
Asterisk needs no additional hardware for Voice-over-IP. A single (or multiple) VOIP provider(s) can be used for outgoing and/or incoming calls (outgoing and incoming calls can be handled through entirely different VOIP and/or telco providers)
For interconnection with digital and analog telephony equipment, Asterisk supports a number of hardware devices, most notably all of the hardware manufactured by Asterisk's sponsors, Digium. Digium has single and quad span T1 and E1 interfaces for interconnection to PRI lines and channel banks. In addition, single to quad port analog FXO and FXS cards are available and are popular for small installations. Other vendors' cards can be used for BRI (ISDN2) or quad- and octo- port BRI based upon CAPI compatible cards or HFC chipset cards.
Lastly, standalone devices are available to do a wide range of tasks including providing fxo and fxs ports that simply plug into the LAN and register to Asterisk as an available device. Asterisk-based telephony solutions offer a rich and flexible feature set. Asterisk offers both classical PBX functionality and advanced features, and interoperates with traditional standards-based telephony systems and Voice over IP systems. Asterisk offers the features one would expect of a large proprietary PBX system such as Voicemail, Conference Bridging, Call Queuing, and Call Detail Records.
Call Features:
* ADSI On-Screen Menu System
* Alarm Receiver
* Append Message
* Authentication
* Automated Attendant
* Blacklists
* Blind Transfer
* Call Detail Records
* Call Forward on Busy
* Call Forward on No Answer
* Call Forward Variable
* Call Monitoring
* Call Parking
* Call Queuing
* Call Recording
* Call Retrieval
* Call Routing (DID & ANI)
* Call Snooping
* Call Transfer
* Call Waiting
* Caller ID
* Caller ID Blocking
* Caller ID on Call Waiting
* Calling Cards
* Conference Bridging
* Database Store / Retrieve
* Database Integration
* Dial by Name
* Direct Inward System Access
* Distinctive Ring
* Distributed Universal Number Discovery (DUNDi™)
* Do Not Disturb
* E911
* ENUM
* Fax Transmit and Receive (3rd Party OSS Package)
* Flexible Extension Logic
* Interactive Directory Listing
* Interactive Voice Response (IVR)
* Local and Remote Call Agents
* Macros
* Music On Hold
* Music On Transfer
o Flexible Mp3-based System
o Random or Linear Play
o Volume Control
* Predictive Dialer
* Privacy
* Open Settlement Protocol (OSP)
* Overhead Paging
* Protocol Conversion
* Remote Call Pickup
* Remote Office Support
* Roaming Extensions
* Route by Caller ID
* SMS Messaging
* Spell / Say
* Streaming Media Access
* Supervised Transfer
* Talk Detection
* Text-to-Speech (via Festival)
* Three-way Calling
* Time and Date
* Transcoding
* Trunking
* VoIP Gateways
* Voicemail
o Visual Indicator for Message Waiting
o Stutter Dialtone for Message Waiting
o Voicemail to email
o Voicemail Groups
o Web Voicemail Interface
* Zapateller
Computer-Telephony Integration
* AGI (Asterisk Gateway Interface)
* Graphical Call Manager
* Outbound Call Spooling
* Predictive Dialer
* TCP/IP Management Interface
Scalability
* TDMoE (Time Division Multiplex over Ethernet)
o Allows direct connection of Asterisk PBX
o Zero latency
o Uses commodity Ethernet hardware
* Voice-over IP
o Allows for integration of physically separate installations
o Uses commonly deployed data connections
o Allows a unified dialplan across multiple offices
Codecs
* ADPCM
* G.711 (A-Law & μ-Law)
* G.723.1 (pass through)
* G.726
* G.729 (through purchase of commercial license through Digium)
* GSM
* iLBC
* Linear
* LPC-10
* Speex
Protocols
* IAX™ (Inter-Asterisk Exchange)
* H.323
* SIP (Session Initiation Protocol)
* MGCP (Media Gateway Control Protocol
* SCCP (Cisco® Skinny®)
Traditional Telephony Interoperability
* E&M
* E&M Wink
* Feature Group D
* FXS
* FXO
* GR-303
* Loopstart
* Groundstart
* Kewlstart
* MF and DTMF support
* Robbed-bit Signaling (RBS) Types
PRI Protocols
* 4ESS
* BRI (ISDN4Linux)
* DMS100
* EuroISDN
* Lucent 5E
* National ISDN2
* NFAS
www.digium.com
www.asterisk.org
http://www.voip-info.org/wiki-Asterisk