I answered a forum post on this question but as I couldn’t upload a picture I thought I might as well add this as a blog post and upload the picture to the blog.
The user wanted to create a report of all the phone call activities for a campaign. It was a little confusing because you couldn’t just filter on the phone call regarding field because the regarding field was linked to a campaign activity (and not the campaign).
So I linked to the campaign activity and then inside there I linked to the parent campaign
it is of course a lot easier if you just look at the screenshot below