Order by x date and receive a discount.
Order by x date and receive a free accessory.
Order by x date and receive a free gift.
Order by x date and receive a free upgrade.
Accept an appointment by x date and receive any of the above.
Enter a contest by x date to create the best application with our technology and compete to win a prize.
Contact us by x date and receive a risk free trial of our product.
Order by x date and avoid an upcoming price increase for an established product.
As you can see from the above examples the key aspects of a call to action are including an incentive and deadline in your offer.
Certainly all the other marketing principles apply as well. i.e. targeting the right community, connecting with prospects who have acute/active pain, etc., however, without a call to action one fails to effectively motivate a prospect to engage with you. Adding a call to action will attract the most qualified prospects to your product or service and yield results.
Good selling!