Unfortunately not. You can purchase and add-on, but that will simply act as credit to pay for any calls, texts etc; it won't allow you to use your normal plan without incurring extra charge. You will see from the price guide - https://cdn.smarty.co.uk/files/SMARTY-Price-Guide.pdf - that calls while roaming in Sri Lanka are pretty expensive. You may do better to find a wifi connection and see if it will support a WhatsApp call.
