5.13.2.4. Handling of extension numbers unknown to the exchange 
The sending of extension numbers unknown to the ISDN exchange should
be avoided. In such a case a substitute number (e.g. the attentand console
number) is sent to the exchange. Configure the substitute extension number
using KNMAT AMO with the modification condition ERSNST (Substitute extension
number).
CHANGE-KNMAT:NPI=ISDN, LEVEL=0, ONNO=<OpenScape 4000 node>, DNNO=<Trunk node>, MODCON=REPEXT, NUMEXT=<substitute extension number>;
Assign the class of service DIDBLK to all stations unknown
to the exchange (e.g. all extension numbers beginning with 4):
CHANGE-COSSU:TYPE=COS, COS=14, AVCE=DIDBLK; CHANGE-SCSU:STNO=4000&&4999, COS1=14, COS2=14;
If, for example, the substitute extension number (number
of the attendant console) is 722 1111 the extension number 4711 is substituted
by
- the substitute number 722 1111 (COS DIDBLK is set)
while extension number 5711
- is substituted by 722 5711 (COS DIDBLK is not set). The rule to use the substitute extension number also applies here, however, the last four digits 1111 are overwritten by 5711.