What is a Call To Action?
A call to action or CTA is something on a website that provokes a response from the visitor. For example a "call now" button or "contact us' link would be a call to action.
It is vital that you have regular call to actions throughout your website. Personally I'd have them featured on every page especially the homepage. Provoking an immediate response from a visitor or customer is what you want and will at least provoke an enquiry or some sort of contact.
Unique visitors will only use a CTA if they are interested in what the website has to offer which is why they are so vitally important to how your website performs.