Emergency Procedure.

If a resistance network had an urgent message to relay that could not wait until the next session, its radio operator could use one of 2 quarz crystals for emergency communications. One established a daytime frequency; the other a night time frequency.
Homestation constantly monitored these frequencies. The agent was sure to receive a response to his emergency call exactly one hour and 10 minutes after he sent it.
This procedure was very risky sine the clandestine radio operator had to contact the home station to be sure it had received the message correctly. In a sample case he confirms this through a rapid exchange of service calls.

BLK is the call of the outstation:

From station BLK I have one message for you, What is the quality of my signal? Over

To station BLK, your signal is good, we are ready to receive your message. Over.

The operator taps his message.
Homestation acknowledge receiving it and asks the operator of the outstation to stand by:

To outstation BLK, message received, please stand by. Over

The outstation now has to wait for response from homestation. After sending the message that the outstation has to stand by. The sheet with the urgent message from the outstation is sent to the cipher room. A cipher specialist enters the received cipher groups under the decryption key and with help of the alphabet table restores the groups into plain text. A message of 30 groups (each group contains 5 letter) takes about 20 minutes for deciphering.
The decoded message is now typed on the keyboard of a teletypewriter: at the other end of the line, 30 miles away, an identical machine begins to run. The message arrives at Baker street 25 minutes later after homestation received it.
About 20 minutes later Baker Street sends it reply which tales about minutes. At the home station the message from Baker Street is encrypted en broken down in 5 letter groups again. All in all 50 minutes after asking the outstation's operator to stand by, homestation sends the following message:

To outstation BLK I have one message for you, are you ready to receive? Over

The outstations response is:
I'm ready, over.

After receiving the message, the outstation replies:
I received your message, over and out. (
It would also be possible just to send QSL SK).

Source: Pierre Lorain 'Clandestine Operations' ISBN 0-02-575200-6.

What we still don't know is on which frequency homestation would answer the outstation's call? I think they would have used their so-called Main-Frequency. They had two of those, one for day time and one for night time transmissions.
If this is the case, I wonder how quickly the outstation operator would have been able to find the homestation on his receiver. Since the receiver was not crystal driven.