You are running late and don't want to lose your reservation.
Be sure to call if you're more than 15 minutes behind schedule. Be specific about what is delaying you: the babysitter, traffic, or a work meeting. And be clear on when you expect to arrive. If you are so late that your table has been given away, apologize and ask, "Is there anything you can do for us?". Most restaurants get far more last-minute cancellations than they'd like to admit, so the chances are slim that there will be nothing available for you all night. Many restaurants also have at least one reserve table that they reluctantly bring out for unexpected situations.
See Real Simple's Menu Lingo Defined
If the restaurant truly cannot offer you a table, try eating at the bar, as you'll get a sense of the restaurant's items and the chef's style, and the food might even be cheaper. As a bonus, you can forge a relationship with the staff, increasing your likelihood of getting -- and keeping -- future reservations.
Instant Wine Smarts
Table Manners Refresher Course