[c-nsp] T.38 Fax Relay from 2620XM

[Gregory Boehnlein]

Hello,
	I have a couple of 2620XM units that I am using as PRI to SIP
gateways. I have been trying to get T.38 fax relay working w/ an endpoint.
What I have discovered is that the Cisco is not sending T.38 invite
information in the SDP message.

Call Path
---------
ISDN PRI (ni2) -> 2620XM -> T.38 Device

What I have also found out is that if I force all calls to be treated as
Faxes on the T.38 machine, it sends back a T.38 invite to the Cisco and the
Fax progresses properly and is received. In my ancient telco brain, I am
used to working w/ CNG tones as determining wether to go into FAX receive
mode or not, but from what I can read, Cisco does not use CNG detection on
it's PRI (I find that weird and it seems incorrect). So how the heck does it
know that an incoming call is a Fax or not???

I would assume that the PRI port would work thusly.. 

1. Answer the ISDN call
2. Detect CNG tone
3. Send T.38 Invite in SDP via SIP

Any pointers? Relevant details.. IP's removed to protect the innocent.

Cisco 2620XM (MPC860P) processor (revision 1.0) with 126090K/4982K bytes of
memory.
Processor board ID JAE073107G7
M860 processor: part number 5, mask 2
1 FastEthernet interface
48 Serial interfaces
2 Channelized T1/PRI ports
32K bytes of NVRAM.
32768K bytes of processor board System flash (Read/Write)

Configuration register is 0x2102

flash:c2600-ipvoicek9-mz.124-23.bin


voice service voip 
 signaling forward rawmsg
 fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback none
 sip
  ds0-num

dial-peer voice 2000 voip
 preference 2
 destination-pattern .T
 voice-class codec 10
 session protocol sipv2
 session target sip-server
 dtmf-relay rtp-nte
 fax-relay ecm disable
 fax nsf 000000
 fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback none
 ip qos dscp ef signaling
 no vad

sip-ua    
 disable-early-media 180
 retry invite 3
 retry response 3
 retry bye 3
 retry cancel 3
 timers buffer-invite 1024
 sip-server ipv4:x.x.x.x:5060