The Custom Extensions module provides you with a facility to register any custom extensions or feature codes that you have created in a custom script or dialplan when the PBX doesn't otherwise know about them. This makes the Extension Registry aware of your own extensions so it can detect conflicts or report information about your custom extensions to other modules. Also, this helps prevent you from creating a duplicate extension number somewhere else in the PBX.
For each custom extension, you can define the following:
Define the custom extension number that you want the PBX to be aware of. You will not allowed to use the number for something else.
Give this custom extension a friendly name.
Here you can enter notes on what this custom extension is used for.
When done, don't forget to click the Submit button followed by the red Apply Config button in order to save and apply your changes.