Haven't installed DCS World yet, but with the standalone v1.1.1.1 the tower gives you the "cleared for visual, contact tower" -line when your distance is about 6 nm from the airport. However, if you try to contact the tower immediately after that line, the tower won't answer. The tower will answer only after you are within approx. 5 nm. So, because you have to wait a bit before asking for the clearance, it's pretty easy to forget to contact the tower, and ATC will give you an "automatic" clearance just before landing.