Lancom LC-1823 Manuals page 45

LANCOM VoIP Routers: PBX Functionalities
Chapter 6: Extended functions
In order for call forwarding to be handled by the exchange in combination
with the PBX in the LANCOM Business-VoIP-Router, three entries are required
in the call-routing table:
Called no.
890
891
899

In this way, the speed dial for immediate, "unconditional call forwarding" can
be complemented by other speed dials for "call forwarding on busy" and
"delayed call forwarding" to trigger call forwarding at the exchange.
ISDN
44
Comment
CF switched off
CF switched on
Check CF
For the called number, enter a speed-dial number that is to transmit the
ISDN facilities to the exchange, in this case '890' and upwards. The speed
dial may not contain control characters (* or #).
As target number you enter the ISDN facilities for communication with the
exchange, e.g. *21*0123456789# to activate call forwarding.
Enter the external telephone number as a call-forwarding target just
as you would dial the subscriber from the public telephone network.
Even if you have deactivated spontaneous outside line access and you
have to dial a zero for outside calls, the forwarding target here is
entered without a leading zero.
As the target line, enter 'ISDN' for each as this function only refers to the
exchange in the ISDN network.
Local
LANCOM Business-
VoIP-Router
Target no. Destination line
#21# ISDN
*21*0123456789# ISDN
*#21# ISDN
External
Exchange
in the ISDN network