code: public boolean register(Credential credential) {
      phone.addUpdateCredential(credential);
      if (phone.register(null, 3600) == false) {
        setErrorMessage(phone.format());
        return false;
      }
  
      return true;
    }
docs: This method registers this notify sender as a UA with the proxy/registrar used to create the
  SipPhone passed to this object's contructor.
  
  @param credential authentication information matching that at the server
  @return true if registration is successful, false otherwise. If false, call getErrorMessage()
  to find out why.