Looking for Caller Fowarded ID from T1 Circuit

Newbie to Asterisk and looking for some direction on code changes for a new project I am working on. I would like to modify code to get additional field “REDIRNUM” from T1 D Channel information.

Trap Output from Telcom Provider Traffic during a Forwarded Call to my Asterisk server is below. As you can see the D Channel provides the detail I need (REDIRNUM)
D1 : L3 TX CREF=0004 IE[05]=CALLGNUM (NumType=National NumPlan=ISDN/Telephony[E.164] PresentInd=Allowed ScrnInd=NetworkProvided Digits=636XXXXXXX)
D1 : L3 TX CREF=0004 IE[06]=CALLDNUM (NumType=National NumPlan=ISDN/Telephony[E.164] Digits=404XXXXXXX)
D1 : L3 TX CREF=0004 IE[07]=REDIRNUM (NumType=National NumPlan=ISDN/Telephony[E.164] PresentInd=Allowed ScrnInd=None Reason=Uncond Digits=636XXXXXXX)

Currently the CALLGNUM is passed on to Asterisk as the CALLERID. See details from full log:
[Jan 14 15:01:25] VERBOSE[3611] chan_dahdi.c: – Accepting call from ‘636XXXXXXX’ to ‘404XXXXXXX’ on channel 0/1, span 1
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@ext-did:2] ExecIf(“DAHDI/1-1”, “0 ?Set(CALLERID(name)=636XXXXXXX)”) in new stack
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@macro-user-callerid:1] Set(“DAHDI/1-1”, “AMPUSER=636XXXXXXX”) in new stack
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@macro-user-callerid:3] ExecIf(“DAHDI/1-1”, “1?Set(REALCALLERIDNUM=636XXXXXXX)”) in new stack
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@macro-user-callerid:18] NoOp(“DAHDI/1-1”, “Using CallerID “TIME WARNER TEL” <636XXXXXXX>”) in new stack
[Jan 14 15:01:25] VERBOSE[4267] res_agi.c: dialparties.agi: Caller ID name is ‘XXXXX TEL’ number is ‘636XXXXXXX’
[Jan 14 15:01:25] VERBOSE[4267] res_agi.c: – dialparties.agi: dbset CALLTRACE/3002 to 636XXXXXXX
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@macro-user-callerid:1] Set(“DAHDI/1-1”, “AMPUSER=636XXXXXXX”) in new stack
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@macro-user-callerid:3] ExecIf(“DAHDI/1-1”, “0?Set(REALCALLERIDNUM=636XXXXXXX)”) in new stack
[Jan 14 15:01:25] VERBOSE[4267] pbx.c: – Executing [s@macro-user-callerid:18] NoOp(“DAHDI/1-1”, “Using CallerID “TIME WARNER TEL” <636XXXXXXX>”) in new stack

Any pointers on getting started would be greatly appreciated. I am using Sangoma A101E with the following:
FreePBX 2.5.2.1
asterisk-1.6.1.9
libpri-1.4.10.2
dahdi-linux-2.2.0.2
dahdi-tools-2.2.0
wanpipe-3.4.8
Linux 2.6.18-164.6.1.el5

I looked at /asterisk-1.6.1.9/channels/chan_dahdi.c but am not clear on how to add this variable.

THANKS!