Font Awesome

Add the function to wp_head hook:

add_action( 'wp_enqueue_scripts', 'tu_load_font_awesome' );
/**
 * Enqueue Font Awesome.
 */
function tu_load_font_awesome() {
    wp_enqueue_style( 'font-awesome', '//use.fontawesome.com/releases/v5.5.0/css/all.css', array(), '5.5.0' );
}


<i class="fa fa-phone"></i>