I have come across three GPL based IP-Based PBX systems
They are
- 1> Asterisk
- 2> Call Weaver
- 3> FreeSwith
- 4> Yate
There are GUI for the above Telephone Systems they are
For Asterisk
- FreePBX => http://www.freepbx.org/
- Trixbox => http://trixbox.org/
- Elastix => http://www.elastix.org/
- Askozia => http://www.askozia.com/
- Briker => http://www.briker.org/
- EasyAsterisk => http://www.easyasterisk.org/
For FreeSwitch
- http://www.fusionpbx.com
For Yate
I have not come across any work , If any one know about it please do let me know
For CallWeaver
I have not come across any work , If any one know about it please do let me know