The new SwitchPi GUI just released, it is stable, flexible, fast and easier to use compare to Freepbx and Diguim GUI.
Here are few first views
SSL secured access
The External lines page
The internal phones page
The menu of the PBX configuration page
The menu of the Call Routing page
The menu of the System Configuration
The menu of the Reporting
You can download it from here, use the dd command to burn it to an sd card, and the default IP address is https://192.168.1.98, the default username is admin, and the default password is admin which is required you to change when you first-time login.