WL is automatic...but.
There is simply too much anecdotal evidence that calling works to ignore. Time after time, we see posts where DISers have been on waitlist, call MS to check, and are told "Your waitlist JUST came through!"
Magic does happen -- CALL!
My anecdotal is that I have had NUMEROUS wait lists and all but ONE cleared without me EVER calling. (The one was for AKV Concerige the weekend of the Marathon...

I never call and have probably cleared about 20 waitlists over the years. (One of the joys of "late booking" is that you tend to have to do this more then those of you that plan ahead!)