code: public List<ReferSubscriber> getRefererInfo(SipURI referToUri) {
      List<ReferSubscriber> list = new ArrayList<>();
  
      synchronized (refererList) {
        for (ReferSubscriber s : refererList) {
          if (s.getReferToUri().equals(referToUri)) {
            list.add(s);
          }
        }
      }
  
      return list;
    }
docs: Gets the subscription object(s) associated with the given referToUri. The returned object(s)
  contains subscription state, received requests (NOTIFY's) and REFER/SUBSCRIBE responses, etc.
  for outbound REFER subscription(s) associated with the referToUri.
  
  @param referToUri the referToUri that was previously passed in to SipPhone.refer(). See javadoc
  there.
  @return ReferSubscriber object(s) associated with the referToUri, or an empty list if there was
  never a call to SipPhone.refer() with that referToUri.