Auerswald COMpact 5200 Advanced Information page 256

Phoning
Unsuccessful Outgoing Calls
You will hear the confirmation tone.
Note: If you do not hear a confirmation tone, the callback request has
been rejected by the public exchange.
3. Hang up the receiver.
As soon as the connection is no longer busy, you will be called by the
public exchange for a period of 20 seconds.
Note: A callback will be stored in the public exchange for a duration of
up to 45 minutes. If the called subscriber has not terminated his call until
then, the callback will automatically be deleted.
4. Pick up the receiver to the make the callback.
Note: If you do not pick up the receiver within the 20 seconds during
which the public exchange calls you, the callback request will be deleted.
If you pick up the receiver, the subscriber will be called.
Note: You also have the option of letting the public exchange call you as
soon as one of the B channels on the previously called ISDN connection
is available. If the subscriber for which you have set up the callback is
phoning on the other B channel, this callback will remain unsuccessful.
The feature code (keypad) to execute the function, as defined in the
PBX, may differ from the requirements of the network provider. In case
of malfunctions, ask your network provider for the required feature code
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COMpact 5200/5200R/5500R - Firmware V7.2 - Advanced Information V04 07/2018