iOS is more restrictive. You cannot use Kalakkal font in WhatsApp or iMessage easily. However, you can:
: The Tamil Text Fonts: Stylish Text app on Google Play allows you to generate stylish text directly for social media. The Story of the "Kalakkal" Script Tamil Kalakkal Font Download
: Because it is a non-Unicode font, you may need a transliteration tool like iOS is more restrictive