After Completion of call you will get a Post Feedback page to Update Call disposition. Click on SMS Template

Click on WhatsApp icon of the Template which you want to send.

Add a Country code and There are three options
Prefix for Current Number : If you want add country code only for current number then click on this.
Prefix for Whole List : If you want add same country code to Entire List then click on this.
Ignore & Continue : If you want to ignore the country code then click on this.

It will Redirect to WhatsApp, if that contact having WhatsApp.