Fax-SIP Troubleshoot Guide

Publisher

Cisco Systems

URL

https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/118647-technote-sip-00.html

Published

2015-01-14

Updated

unknown

Accessed

2026-10-02

HTTP status

200

License

Cisco proprietary; all rights reserved; no-redistribution; short excerpts and locators only

Source notes citing this source

• Cisco gateways support only G3 fax over T.38; on an SG3 call the originating gateway squelches the V.8 CM tone so the SG3 machines fall back to G3. in context
• In Cisco topologies that include Unified CM, the T.38 switchover first sends a T.38 offer with a 0.0.0.0 address and later another T.38 re-INVITE with the real IP address. in context
• Cisco's fax troubleshooting debugs include debug fax relay t30 (all-level-1 or all), debug ccsip, debug voice ccapi inout and debug voip vtsp all. in context
• ECM is optional for G3 fax but mandatory for Super G3 (V.34) fax. in context
• When a fax relay call shows Failure To Train (FTT), Cisco advises checking T1/E1 digital interfaces for clocking errors and slips. in context
• Cisco's Fax-SIP troubleshooting guide gives network limits for T.38 of delay under 1000 ms and jitter under 300 ms. in context
• On Cisco IOS voice gateways using protocol-based T.38, the terminating gateway initiates the switchover from voice to T.38 by sending the re-INVITE once it detects the V.21 preamble flags from the answering fax. in context

Cite this source record

APA

WarmTransfer. (2015, January 14). Fax-SIP Troubleshoot Guide. WarmTransfer. https://warmtransfer.net/knowledge/sources/cisco-technote-118647-fax-sip-troubleshoot

BibTeX

@misc{warmtransfer-cisco-technote-118647-fax-sip-troubleshoot,
  title  = {Fax-SIP Troubleshoot Guide},
  author = {{WarmTransfer}},
  year   = {2015},
  url    = {https://warmtransfer.net/knowledge/sources/cisco-technote-118647-fax-sip-troubleshoot},
  note   = {Cisco Systems, accessed 2026-10-02}
}