function default_comments_on( $data )
{
$myCustomPostType = array("my_custom_post_type_1", "my_custom_post_type_2", "contacts"); /* Custom Post Type Array */
if (in_array($data['post_type'], $myCustomPostType))
{
$data['comment_status'] = 'open';
}
return $data;
}
add_filter( 'wp_insert_post_data', 'default_comments_on' );
Add this code in functions.php