If I understand well,
when you are in the function hooked by :
add_filter('acf/save_post' , 'fn_save_post', 10, 1 );
your post and acf values are already saved, so you can get them by using :
$your_acf_field_key = get_field('your_acf_field_key', $post_id);
then, you can deal with all variables to send SMS or whatever
