The PBX Software System provided by MAS World Development has an impressive range of features (listed below in an alphabetical order)

Authentication
Automated Attendant
BlacklistsBlind Transfer
Call Detail Records
Call Forward on Busy
Call Forward on No Answer
Call Forward Variable
Call Monitoring
Call ParkingCall Queuing
Call Recording
Call Routing (DID & ANI)
Call Transfer
Call Waiting
Caller ID
Caller ID Blocking
Caller ID on Call Waiting
Conference Bridging
Database Store / Retrieve
Database Integration
Dial by Name
Direct Inward System Access
Do Not Disturb
Fax Transmit and Receive
Flexible Extension Logic
Interactive Voice Response (IVR)
Local and Remote Call Agents
Music On Hold
Music On Transfer:

Flexible Mp3-based System
Random or Linear Play
Volume Control

Privacy
Protocol Conversion
Remote Call Pickup
Remote Office Support
Route by Caller ID
Spell / Say
Supervised Transfer
Talk Detection
Text-to-Speech
Three-way Calling
Time and Date
Transcoding
Trunking
VoIP Gateways
Voicemail:

Visual Indicator for Message Waiting
Voicemail to email
Voicemail Groups