code: public static boolean isSystemHeaderAndNotGruu(ModifiableRule modifiableRule, Parameterable parameterable) {
  		boolean isSettingGruu = false;
  		if(modifiableRule == ModifiableRule.ContactSystem &&
  				(parameterable.getParameter("gruu") != null ||
  					parameterable.getParameter("gr") != null)) {
  			isSettingGruu = true;
  			if (logger.isDebugEnabled())
  				logger.debug("Setting gruu so modifying contact header address is allowed");
  		}
  		return !isSettingGruu && isSystemHeader(modifiableRule);
  	}
docs: Support for GRUU https://github.com/Mobicents/sip-servlets/issues/51
  if the header contains one of the gruu, gr, temp-gruu or pub-gruu, it is allowed to set the Contact Header
  as per RFC 5627