IP PBX using FreePBX Solution on Ubuntu

FreePBX is a web-based open source GUI (graphical user interface) that controls and manages Asterisk (PBX), an open source communication server. FreePBX is licensed under the GNU General Public License (GPL), an open source license. This solution is a pre-configured FreePBX Distro that includes the system OS, Asterisk, FreePBX GUI and assorted dependencies.

FreePBX Features
Add or change extension and voicemail accounts in seconds
Native support of SIP, IAX, and ZAP clients and more
Supports all Asterisk supported trunk technologies
Reduce long distance costs with LCR
Route incoming calls based on time-of-day
Create interactive Digital Receptionist (IVR) menus
Design sophisticated call groups
Manage callers with Queues
Upload custom on-hold music (MOH)
Search company directory, based on first or last name
Detect and receive incoming faxes
Share administrative duties
Backup and Restore your system
Save audio recordings of calls
View extension and trunk status with Flash Operator Panel
View conversation recordings with Asterisk Recording Interface (ARI)
Unlimited number of Voicemail boxes
Follow Me functionality
Ring Groups with calls confirmation
Unlimited number of Conferences
Music on Hold (via MP3s, or streamed off the internet)
Paging and Intercom functionality for man SIP phones that support it
Call Queues

Disclaimer: FreePBX is a registered trademark of Sangoma Technologies Inc and is licensed under the GPL.