Calls to action may vary. You may want your visitors to subscribe, to register, to buy something, to bookmark your post, etc. With calls to action everything is important: the color of your buttons and supporting elements, the language you use to describe the action, the place on the page where you locate them, the additional elements that encourage people to act. All these elements should be consistent and support each other getting your visitors one step closer to their objective.
Link: How To Call To Action
More often than not, it is not enough for you that your visitors just come and go. You want them to do something at your site. Unfortunately usually webmasters pay too little attention to their calls-to-action focusing on design, content and SEO. It is sad to see truly good resources that are doing great in SERPs but fail to engage their visitors.