First off, my sincerest of apologies if I am posting something here that has been 1) already answered and I've ever so blindly missed 2) something that is painfully simple and I am just waaay overthinking it or 3) do anything in this post that could generate an onslaught of people to virtually tar and feather me forum style.
Okay, now that is out of the way, on to the good stuff. I've been reading this forum for months now, as well as googled till I can google no more in search of the ellusive answer to (what seems to be?) getting Inbound T.38 faxing from my ITSP (Callcentric) to my Exchange UM Server, and on to the destined mailbox with my beloved PBXnSIP (loved more and more each day!) sitting right in the middle of ITSP and Exchange.
I feel as if I am very close, either that or my fear will shortly be proven and I have very little idea what the heck is actually going on. However, When sending a fax from the PSTN (310) 305-1073, to a DID on my callcentric account (310) 496-7515, Callcentric delivers the call as expected, PBXnSIP (localhost) sees it, sends the INVITE to across my Exchange UM trunk to the Exchange UM Server (oim-lax-ums-002), where I have added as a secondary fax-enabled extension 7515 (and faxing is enabled on the Exchange dial plan as well), the voicemail box picks up, Exchange says hey you aren't talking, you're beeping in CNG, I need to change. At that point, I think it looks like everything switches over to T38, and then just like magic, boom, absolutely nothing happens.
I can hear the voicemail greeting thru the PSTN fax on via the speaker.
<Simulation of Voice> "Hello this is me, I'm a voicemail box, please oh please let the fax just work!" </the end>
I can see the T38 switch happening (I think) from the logfile (see below)
SIP/2.0 200 Ok
Via: SIP/2.0/UDP 126.96.36.199:5080;branch=z9hG4bK-dcbcf5a31b41b4dedebcd0e68c48fd79
CSeq: 1 INVITE
Supported: 100rel, replaces, norefersub
Allow: INVITE, ACK, CANCEL, BYE, REFER, PRACK, INFO, UPDATE
o=- 23537 23537 IN IP4 192.168.12.129
c=IN IP4 192.168.12.129
m=audio 56518 RTP/AVP 0 101
 2008/10/31 06:27:37: SIP Rx udp:188.8.131.52:5080:
ACK sip:firstname.lastname@example.org:5060;transport=udp SIP/2.0
v: SIP/2.0/UDP 184.108.40.206:5080;branch=z9hG4bK-d07ae536da0612b3ab058fe9274dafa8
CSeq: 1 ACK
 2008/10/31 06:27:38: SIP Rx tcp:192.168.12.114:5065:
INVITE sip:email@example.com:57932;transport=tcp SIP/2.0
CSEQ: 1 INVITE
VIA: SIP/2.0/TCP 192.168.12.114:5065;branch=z9hG4bKf31bc4d2
o=- 0 1 IN IP4 192.168.12.114
c=IN IP4 192.168.12.114
m=audio 0 RTP/AVP 0 8 101 13
m=image 55320 udptl t38
I can read that Exchange got a call that it thought was a fax, by looking at the Event Log on the UM Server, curiously saying no pages were received (see event log item directly below):
Log Name: Application
Source: MSExchange Unified Messaging
Date: 10/31/2008 6:28:26 AM
Event ID: 1028
Task Category: UMCore
The fax call from "7515" for "Jeremiah L. Maudlin" at number "7515" was received but no pages were received. The fax call was 48.7017504 seconds long. The fax call was received from UM IP gateway "192.168.12.126".
<Provider Name="MSExchange Unified Messaging" />
<TimeCreated SystemTime="2008-10-31T13:28:26.000Z" />
<Data>Jeremiah L. Maudlin</Data>
Alas, no fax in my inbox, an agry PSTN fax machine that says "would you give it up already!" and starts beeping at me, and one confused IT guy sitting at his desk shaking his head and ever so hopeful that a forum post will lead to his rescue.
A couple other quick items of note:
Exchange 2007 SP1 UM, CA, MB, and HT roles all on 4 seperate servers
PBXnSIP 3.0.3037 w/ Permanent License, 3 Trunks, a number of accounts, 1 Dial Plan
Fully OCS Integrated w/o issue (horray! thanks to pbxnsip dev & Jan B. for excellent work!)
Callcentric SIP Trunk w/ ~30 DIDs
Exchange globalcfg.xml file changes:
(As for two items above, I have tried various combinations of true/false, and this seems to get me closest to success)
I've also attached the complete PBXnSIP log (Level 9) to this post for review from a failed Fax call just a few moments ago. Thanks in advance to all, it is most appreciated!