Hey you did you fixed it?
the new code seems like a confusion.
but here is the new work arround
remove_action('wp_head', 'lead_call_button_font_awesome_css');
add_action('wp_head', 'xxx_lead_call_button_font_awesome_css');
function xxx_lead_call_button_font_awesome_css() {?>
<link href="<?php echo WP_PLUGIN_URL . '/' . plugin_basename( 'lead-call-buttons' ) . '/'.'css/plugin-main.css'?>" rel="stylesheet"/>
<link href="//netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.css" rel="stylesheet"/>
<?php
}
Please remove the HTTP in front of the //netdna.bootstrapcdn.com