Source note · Troubleshooting fax over IP failures
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.
Checked 2026-10-02
Once it detects the V.21 Flag, it initiates switchover.
- Vendor
- Cisco
- Product
- Cisco IOS voice gateways
- Subsystem
- T.38 switchover
- Deployment
- on-premises
- Region
- not restricted
- Release range
- as documented 2015
- Checked
- 2026-10-02
Sources
- Fax-SIP Troubleshoot Guide — Cisco Systems · tier 2 current vendor documentation · T.38 fax relay call flow section (TGW switchover on V.21 flag)
Cite this note
APA
WarmTransfer. (2026, October 2). Troubleshooting fax over IP failures: source note fax-over-ip-troubleshooting-tgw-sends-reinvite. WarmTransfer. https://warmtransfer.net/knowledge/claims/fax-over-ip-troubleshooting-tgw-sends-reinvite
BibTeX
@misc{warmtransfer-claim-fax-over-ip-troubleshooting-tgw-sends-reinvite,
title = {Troubleshooting fax over IP failures: source note fax-over-ip-troubleshooting-tgw-sends-reinvite},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/fax-over-ip-troubleshooting-tgw-sends-reinvite},
note = {Source note fax-over-ip-troubleshooting-tgw-sends-reinvite, checked 2026-10-02}
}