Oooh, you're using PRIs. Have you taken a look at your ISDN public-unknown numbering?
First:
PRI trunk group, page 1 -- numbering format is set to "pub-unk"
page 2 -- send name is set to "N" (business names are often unclear when sent by the T1 provider), send calling number is set to "Y". and numbering format is set to "public."
Then:
Change isdn public-unknown-numbering (ch isdn pub)
enter individual extension or extension block
With a 5-digit dial plan, you might use:
"3" for 3XXXX
"367" for 367XX extensions
"36507" for an individual extension
Enter the ISDN-PRI trunk groups (standard is to enter 26-27 for PRI trunk groups 26 and 27)
Enter the CPN prefix -- the number you want the called party to see when one of these extensions completes an outside call -- "8314620777" to send the enter number, or "831462" if you want to send the individual DID number
Enter CPN length as 10 (digits)
Hope this is helpful!