Modem calls through VG320 Fail

[jasunf]

Hello all!

I have a challenge I've been working on, searching for documents, reading documents, open TAC case, etc.  

Challenge is I'm trying to put a modem on a VG320 FXS port and call outbound through the SIP gateway to save on toll charges.  I have a USB dongle modem I put on a pots line and call my test number which gives me the challenge for username password.  When I put the modem on the VG320 it just times out ringing.  

I'm figuring I need to do pass through as relay requires two voip gateways end to end.  I'm trying to send the call out of a voip gateway into a plain old pots line at the other end (is that even supported).

Call path is: Laptop Modem -> VG320 FXS -> Cisco 4331 Sip Cube -> Provider SBC -> POTs line modem (no gateways)

Attached are the configurations of the two gateways with sensitive and useless information removed.  The gateway is VG320 is SCCP in call manager.  Debug show the outbound dial peer of the SIP gateway is 2000 (as noted in the config).  

This has dragged on for weeks and I'm looking for any kind of direction.  Thank you all

[Brandon Buffin]

As you're probably aware, fax/modem calls are very susceptible to any network problems (jitter, delay, packet loss, etc.). Make sure you have ruled out any bandwidth problems. Also, you can try decreasing the speed on the modem to 9600 or 14400.

Brandon

[Scott Pedersen]

Did you enable modem passthrough on the gateway in cucm?

Also would add modem pass-through on the SIP gateway under "voice services voip" command is "modem passthrough" I would look at the following document to determine what works best for your environment: 

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/fax/configuration/15-mt/vf-15-mt-book/vf-cfg-mdm-psthr.pdf

[jasunf]

Hey guys thanks for the replies.  

Yes we have modem pass through enabled on the gateway.  It is setup in SCCP mode from CUCM to VG320.  Below is a snip from our gateway config:

I believe adding the modem pass through on the sip gateway was tried by a TAC engineer without success.  However I do not see it in the config.  I will re-enable it and try a test call. 

Thanks again!

[jasunf]

FYI for anyone else who runs across this and is trying to modem passthrough from a vg320 to SIP gateway the fix was this on the VG320:

voice service voip
fax protocol none
no fax-relay sg3-to-g3 <--- adding this line resolved our issue

Thanks to all who replied.  

Special Thanks to Robert Cash with Cisco TAC who mentioned that above fix.