The calls and the SMS missed call notifications will cost you credits.
To see how many communication credits you have left, click on Account => Account Settings
You’ll land on the Payments and Budget page, where you’ll see the “Your communication Credits” section. There your can see your credits and if you pass the mouse over the “Learn more” link, you’ll see the fees for your country.
To add credits, click on the "Add credits" button and follow the steps.