If you want to register to a trunk without authorization you need to:
- Edit the trunk in question
- Set Register option to not required
- Set Insecure option to very
- Set Username field to anything you want (PBXware GUI requires something there)
- Save the trunk
If you want to use Multi user extension as a connection to another trunk you need to:
- Edit the extension in question
- Click on Advanced Options
- Scroll down and click on Additional config
- Add the following to the list:
insecure=very
permit=X.X.X.X
(where X.X.X.X is the IP address which is used for registering)