public interface Strategy {
   
  /**
   * Add a phone to the conversation
   * @param fromPhone a phone already part of the 
   * conversation
   * @param toPhone a phone to be added to the 
   * conversation
   */
  public void makeCall( Phone fromPhone, Phone toPhone );
  
  /**
   * Hang up the phone
   * @param phone the phone hung up
   */
  public void hangUp( Phone phone );
  
  /**
   * Return a string representation of the conversation
   */
  public String toString();
  
}