#1. Add this code to the Code Injection >Header or Code Block

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.1.1/css/all.min.css" integrity="sha512-KfkfwYDsLkIlwQp6LFnl8zNdLGxu9YAA1QvwINks4PhcElQSvqcyVLLD9aMhXd13uQjoXtEKNosOWaZqXgel0g==" crossorigin="anonymous" referrerpolicy="no-referrer" />

#2. Add this code to Code Injection > Footer

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.0/jquery.min.js"></script>
<script>
$(document).ready(function(){
$('<a href="tel:0123456789" class="mobile-phone"><i class="fa-solid fa-phone fa-beat"></i></a>').appendTo('.header-burger');
});
</script>

You will have

#3. Use this code to Custom CSS box

div.header-burger {
    flex-direction: row-reverse;
    align-items: center;
}
.mobile-phone {
    position: relative;
    left: -20px;
    top: -5px;
}

Result: