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.