code: public Iterator<String> getChallengeRealms() {
  		List<String> realms = new ArrayList<String>();
  		final Response response = getResponse();
  		if(response.getStatusCode() == SipServletResponse.SC_UNAUTHORIZED) {
  			WWWAuthenticateHeader authenticateHeader = (WWWAuthenticateHeader) 
  				response.getHeader(WWWAuthenticateHeader.NAME);
  			realms.add(authenticateHeader.getRealm());
  		} else if (response.getStatusCode() == SipServletResponse.SC_PROXY_AUTHENTICATION_REQUIRED) {
  			ProxyAuthenticateHeader authenticateHeader = (ProxyAuthenticateHeader) 
  				response.getHeader(ProxyAuthenticateHeader.NAME);
  			realms.add(authenticateHeader.getRealm());
  		}
  		return realms.iterator();
  	}
docs: /*
  (non-Javadoc)
  @see javax.servlet.sip.SipServletResponse#getChallengeRealms()