Asterisk is a complete PBX in software. It runs on Linux and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in three 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 and H.323 (as both client and gateway). Check the Features section for a more complete list.

Asterisk needs no additional hardware for Voice over IP. 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 as well as a single port FXO card and a one to four-port modular FXS and FXO card.

• 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
  • Flexible Mp3-based System
    
  • Random or Linear Play
    
  • 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
  • Visual Indicator for Message Waiting
    
  • Stutter Dialtone for Message Waiting
    
  • Voicemail to email
    
  • Voicemail Groups
    
  • Web Voicemail Interface
• Zapateller

• AGI (Asterisk Gateway Interface
  
• Graphical Call Manager
  
• Outbound Call Spooling
  
• Predictive Dialer
  
• TCP/IP Management Interface

• TDMoE (Time Division Multiplex over Ethernet)
  • Allows direct connection of Asterisk PBX
    
  • Zero latency
    
  • Uses commodity Ethernet hardware
• Voice-over IP
  • Allows for integration of physically separate installations
    
  • Uses commonly deployed data connections
    
  • Allows a unified dialplan across multiple offices

• 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

• IAX™ (Inter-Asterisk Exchange)
  
• H.323
  
• SIP (Session Initiation Protocol)
  
• MGCP (Media Gateway Control Protocol
  
• SCCP (Cisco® Skinny®)

• E&M
  
• E&M Wink
  
• Feature Group D
  
• FXS
  
• FXO
  
• GR-303
  
• Loopstart
  
• Groundstart
  
• Kewlstart
  
• MF and DTMF support
  
• Robbed-bit Signaling (RBS) Types

• 4ESS
  
• BRI (ISDN4Linux)
  
• DMS100
  
• EuroISDN
  
• Lucent 5E
  
• National ISDN2
  
• NFAS