Creating a telephone link is much like creating a mailto: link...
your link would look like this:
<a href="tel:1234561234">Call ME!</a>
this tells browser plugins and smart phones that the link being followed is a telephone number and to use the default telephone application installed.
EDIT:
Based on feedback and what other users have commented, I would suggest trying this, I am unsure if this will work as expected but maybe try some different combinations.
<a href="whatsapp://tel:3216541234"></a>
Or
<a href="whatsapp://3216541234"></a>
if this works for you please mark the other user who posted this information as the right answer... I simply referenced his/her content, I knew this kind of thing existed but did not know the url design so they do deserve the credit.
SECOND EDIT:
Visit this website to see kind of how this should work: https://www.whatsapp.com/faq/android/28000012
based on what I see here, I would actually guess it's more like:
<a href="whatsapp://call?number=##########"></a>
Please let us know the working result so I can modify my response to have the right answer. :)
