Troubleshooting T.38 Fax over IP (FoIP) and Voice errors requires identifying which stage of the call is breaking down: Voice Media, Switchover, or Fax Media. Because T.38 faxes always begin as a standard voice call before transitioning to fax mode upon detecting fax tones, structural network performance and signaling configuration directly dictate transmission success.
Below is a technical guide to diagnosing and fixing the most frequent points of failure. 1. SIP Re-INVITE & Switchover Failures
This error occurs when the initial voice call connects, but the system fails to transition to T.38 mode.
The Cause: T.38 requires a mid-call SIP re-INVITE to shift media streams from voice to UDPTL (UDP Transport Layer). Firewalls, Session Border Controllers (SBCs), or aggressive Network Address Translation (NAT) often block these re-INVITE requests. The Fix:
Verify your firewall rules explicitly allow SIP re-INVITE actions.
Disable SIP ALG (Application Layer Gateway) on all intermediary routers, as it frequently corrupts SIP packet headers.
Ensure the designated UDPTL media ports are open and properly routed end-to-end. 2. Network Impairments (Packet Loss & Jitter)
Faxes are incredibly intolerant of timing delays, resulting in dropped calls or completely unreadable, garbled pages.
T.38 Fax Protocol Explained: How It Works (2026 Guide) – mFax
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