JSFiddle - React, Tailwind, and code Playground
HTML
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<a href="#" class="phone_number" id="favorite" onclick="Fav()" >
<i class="fa fa-heart"></i>
</a>
JavaScript
var btnvar = document.getElementById('favorite')
function Fav() {
if(btnvar.style.color == "red"){
btnvar.style.color = "grey"
}
else{
btnvar.style.color = "red"
}
};