Creating a telephone link is much like creating a mailto: link...
your link would look like this:
Call ME!
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.
Or
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:
Please let us know the working result so I can modify my response to have the right answer. :)
