Hi,
I guess we would need to know what specific board or product you are using in this case. But I did manage to see further detail in the GC ISDN Tech guide here:
More specifically this exert from section 4.11:
4.11 Using Direct Layer 2 Access
When using Dialogic® Springware Boards, to activate layer 2 access, parameter 0024 in the firmware PRM file must be set to a value that includes the bit represented by the value 01H. See Table 26, “Modifiable Protocol Parameters for Dialogic® Springware Boards”, on page 192 for more information.
When using Dialogic® DM3 Boards, direct layer 2 access is supported on a per trunk basis. Direct layer 2 access is enabled by including the following command in the appropriate [CSS.x] section of the CONFIG file, where x identifies a specific trunk (span):
Setparm=0x9,1
...
Regards,
Jeff