No good to call unless you like being on hold for hours.
Best bet is call immediately when they open.
However, they will only tell you a stories...fraud protection etc.
BUT, do ask them if they are going to pay you interest for the duration that they have been holding your refund back. After all they would charge you interest in a heart beat, albeit about 2 years of interest, which is how long they wait to notify you of errors on your refund.
