Under the "PBX" tab, select "PBX Configuration" from the drop-down menu.
Under the "Basic" category, select the "Extensions" option.
To select your extension type, select "Generic SIP Device" from the drop-down menu, then click on the "Submit" button.
There are a lot of different settings here to choose from, but we are just making a simple extension. Please review the settings and change as needed for your scenario.
Submit changes after each extension, once all are built select "Apply changes"