private void PhoneNumbersCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.OldItems != null)
            {
                foreach (var item in
                    e.OldItems.Cast<PhoneNumber>().Select(p => PhoneNumbers.First(i => i.Item == p)))
                {
                    PhoneNumbers.Remove(item);
                    item.Dispose();
                }
            }

            if (e.NewItems != null)
                e.NewItems.Cast<PhoneNumber>()
                    .ForEach(p => PhoneNumbers.Add(new StaffingResourcePhoneItemViewModel(p, EditMode)));

            EnsureDelete();
        }