You can subscribe to the PreviewKeyDown event of the control. In the event handler, you can double check if all the text is select and if the delete key is pressed. If yes, you can execute the clear command of the control.

private void TxtPhone_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            var myMask = sender as RadMaskedTextInput;
            if (e.Key == Key.Delete && myMask.Text.Length == myMask.SelectionLength)
            {
                myMask.ClearCommand.Execute(null);
            }
        }