In the SS7 IC for Voice Gateway this function is performed on the NAS to initiate the call back to the PSTN (VOIP to PSTN direction -
The other direction is chosen by the SSP in the PSTN). You can set B channel to ascending or descending in the NAS.
You do this under the serial interface of the D channel:
conf t
int ser 0:23
isdn bchan-num-order
By default the NAS uses descending. (Controller 3 2 1 0)
When coding ascending it uses controller 0 1 2 3. It uses
DS0s 1-24 with ascending and 24-1 for descending.
