Null Value - Unanswered Phone Calls missing

I have a table of phone call facts, that includes a foreign key to a users table.  When making dashboards, things have been going well as long as the phone call is answered - because the user_id field is populated and aligns with a worker.  Unfortunately, some calls are not answered so the user_id for that row’s column is null.

Once I add the full_name to the insight, all of the rows missing a name due to the foreign key disappear (but are still included in the roll up).  I’ve referenced the how-to guide but something isn’t clicking for me how how to cater this to my call center use-case.

Can anyone access my account (admin has access from prior ticket) to tell me what I may be doing wrong?  This is a stopper for our migration.