Sri Lanka?

Nope, Tesco International Calling app can use your Smarty number when registered and you can send free texts internationally using your Smarty number when you have topped up (£3 minimum top up at a Tesco checkout, £5/10/20 online/app). Incoming texts will go to your usual message inbox as the Tesco Calling app sends texts with your (Smarty) registered number. 

It’s a great work around roaming for any network, data off and use Tesco app texting via wifi or a local sim hotspotting on another phone MiFi, or put your Smarty sim in a basic 3/4G phone and put a SL sim in your regular mobile. 

Hi Nichola_1808396 . I tried to answer this on your previous post.

In case you did not see my reply, here it is again:

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.