Protocols
IAX (Inter-Asterisk Exchange)
SIP
H.323
MGCP
SCCP (Cisco Skinny)

Codecs
G.729
G.723.1
G.711 (A-Law & μ-Law)
G.726
G.722
iLBC
GSM
ADPCM
Linear
LPC-10
Speex

PRI Protocols
BRI (ISDN4Linux)
EuroISDN
DMS100
Lucent 5E
National ISDN2
NFAS4ESS

Computer-Telephony Integration
TCP/IP Management Interface
Graphical Call Manager
Outbound Call Spooling
Predictive Dialer
AGI (Asterisk Gateway Interface)

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

Traditional Telephony Interoperability
FXO
FXS
MF and DTMF support
E&M
E&M Wink
Feature Group D
GR-303
Loopstart
Groundstart
Kewlstart
Robbed-bit Signaling (RBS) Types
MFC-R2

Call features
ADSI On-Screen Menu System
Alarm Receiver
Append Message
Authentication
Automated Attendant
Blacklists
Blind Transfer
Call Detail Records
Call Forward on No Answer
Call Forward Variable
Call Forward on Busy
Call Transfer
Call Waiting
Call Monitoring
Call Queuing
Call Routing (DID & ANI)
Call ParkingCall Retrieval
Call Recording
Call Snooping
Caller ID
Caller ID on Call Waiting
Caller ID Blocking
Calling Cards
Conference Bridging
Database Store / Retrieve
Database Integration
Dial by Name
Direct Inward System Access
Distinctive Ring
Distributed Universal Number Discovery
Do Not Disturb
E911
ENUM
Fax Transmit and Receive
Flexible Extension Logic
Interactive Directory Listing
IVR(Interactive Voice Response)
Local and Remote Call Agents
Macros
Music On Hold
Music On Transfer:
- 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
Zapateller