A call to action simply means you are alerting your friends that you have a new story created on snap chat and you would like them to view your story. It is like a commercial or a promotion to help increase your snap chat views by getting more people to view them. You should however be careful not to end up spamming your friends as most people delete spam messages before even reading them.
You can use CTA by sending a snap to all your snap chat friends. It should short and simple especially if you have a lot of friends as you may be required to redo it. To increase the snap chat views further, you can share the snap or video clip to other social media sites such as twitter and face book.
You can send the CTA before you create the snap chat story or immediately after you do so. Your message should have an urgent tone in it. The aim of this is to get people interested in seeing your story before the twenty four hours are over and it gets permanently deleted. Doing it before creating the story gets people to know that a cool story is coming up on snap chat and they should be ready for it.