$merge_tags; issue

I have absolutely not understood how to show a merge_tag in mailchimp contacts. If the attached code worked, should I immediately see the TEL field populated in mailchmp? Do you need to do a resync? I do not understand!!

function mailchimp_custom_order_merge_tags($merge_tags, $order) {

    /// add whatever you want to the merge tags
    $merge_tags['TEL'] = 'test';

    return $merge_tags;
}

add_filter(‘mailchimp_get_ecommerce_merge_tags’, ‘mailchimp_custom_order_merge_tags’, 10, 2);