Recommendation U.21
OPERATOR RECALL ON A TELEX CALL SET UP
ON A RADIOTELEGRAPH CIRCUIT
(New Delhi, 1960; amended at Geneva, 1964)
	The CCITT,
considering
	(a)	that experience has shown that, for telex calls set up over a radiotelegraph circuit, it was useful to enable the telex subscriber to cause an operator to re–enter on a call in progress without interrupting it;
	(b)	that such re–entry may be of interest in the following cases as well as in the case of a defective connection:
i)	When a subscriber decides, in the course of a call, to change from a plain text to a cypher he can call the operators in the terminal radio exchanges and ask them to interrupt the delay signal, which might otherwise disturb the synchronism between the cyphering apparatus used at the two ends.
ii)	When a subscriber has sent a message but waits a very long time for a reply from his correspondent, he can ask the operator whether his message is still being stored or whether it is expected that any interruption to the radio circuit will continue. If need be, he can then choose another means of communication (telegram or telephone call) to send an urgent message to its destination;
	(c)	that although it seems that re–entry by an operator will be limited mainly to national networks (for example by a subscriber calling the controlling telex operator on the radiotelegraph circuit), international standardization of an operator recall signal would be useful if the controlling telex operator on the radiotelegraph circuit is located in a transit country, and also for intermediate manual switches; this would no doubt prove to be a great advantage when this possibility is generally utilized,
unanimously declares the following view
	(1)	If the Administrations concerned agree on the use of a special signal enabling a subscriber to recall an international telex operator's position making use of radiotelegraph circuits, such a recall must not cause release of a call in progress.
	(2)	This operator recall signal will consist of the following sequence: combinations No. 28 (line–feed) followed by four combinations No. 27 (carriage–return).
	(3)	The detection device causing re–entry by the operator will be controlled by the receipt of four consecutive combinations No. 27; combinations No. 28 will only be used to avoid superposition of the text on the receiving teleprinter and will not have to be recognized by the detection device.
	(4)	The equipment for discriminating the operator recall signal will be switched off by a sequence of four consecutive combinations No. 19 (signal for transfer to data).