If you want to register to a trunk without authorization you need to:

1. Edit the trunk in question
2. Set Register option to not required
3. Set Insecure option to very
4. Set Username field to anything you want (PBXware GUI requires something there)
5. Save the trunk

If you want to use Multi user extension as a connection to another trunk you need to:

1. Edit the extension in question
2. Click on Advanced Options
3. Scroll down and click on Additional config
4. Add following to the list:

     insecure=very
     permit=X.X.X.X
     

1. Save the extension