First you have to verify that the ISDN-PRI software is turned on in your switch. Do 'display system customer-options'. Hopefully the ISDN-PRI field is set to 'y'. For a G3siV6 you can do ISDN-PRI one of two ways; use a TN767 and a port on a Processor Interface board (TN765) or use a TN464 and the PACCON feature. The second option requires a TN778 Packet Interface board and the feature to be enabled on the Customer Options form.
After determining which method you will use, install the hardware and add the DS-1 board to the system (add ds1 xxxx). It should be set for B8ZS, ESF, and ISDN-PRI. Depending on the type of CO you're connecting to determines Protocl field gets set to; a=4ESS, b=5ESS, c=DMS250.
Next add the Signaling Group. The 24th channel is used for signalling.
Finally, build the Trunk Group, type isdn. Add the members and reference the Signaling Group created above.
Kevin