D7 update field in database problem
If you want to update your field when user was deleted you should put your update query into your sms_user_delete. Because userfieldmobil_user_update will only works if u update user data, not delete it.

function sms_user_delete($uid, $number = FALSE) {
  userfieldmobil_user_update($account-&gt;uid); // my custom field about users i call this manually. 
  $query = db_delete('sms_user')
    -&gt;condition('uid', $uid);
  if ($number) {
    $query-&gt;condition('number', $number);
  }
  $num_deleted = $query-&gt;execute();

  $num_updated = db_update('field_data_field_mobilnumber')
     -&gt;fields(array(
     'field_mobilnumber_value' =&gt; '',
     ))
     -&gt;condition('entity_id', $uid, '&gt;=')
     -&gt;execute();
  }


Its not like in drupal 6 where u can use hook_user, read about hook_user_update and hook_user_delete for more info.