Whatsapp Intent doesn't work

I've used opening of Whatsapp from within an Ionic app in the past.
Your config.xml should contain:
<access launch-external="yes" origin="whatsapp:*"/>

There's no need to use ng-href here, no value to be evaluated. Does it work when you alter your config.xml like above and use the following snippet:

<a class="button button-clear button-large" href="whatsapp://send?text=Hello%20World!">
  <i class="icon ion-social-whatsapp t-green"></i> Open WhatsApp
</a>

Good luck.