By David Thompson · Published May 6, 2025 · Updated June 8, 2026 · 8 min read
Quick Answer: T.38 is the ITU standard protocol that makes fax work reliably over IP networks. It converts fax tones into fax-aware data packets — solving the jitter, compression, and packet loss problems that break fax on VoIP. If you'd rather skip VoIP configuration entirely, mFax.to lets you fax from any device with zero protocol setup.
If you have ever tried to fax over a VoIP phone system, you have probably hit the wall: dropped pages, communication errors, and faxes that silently fail. The T.38 fax protocol was designed to solve exactly this problem.
T.38 has been an ITU-T standard since 1998, yet most people — including IT admins — have only a vague sense of what it does. This guide explains the T.38 protocol in plain language: how it works, how it compares to the alternative (G.711 passthrough), and when it actually matters for your fax workflow.
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What Is the T.38 Protocol?
T.38 is an ITU-T recommendation that defines how to transmit real-time fax (Group 3) over IP networks. In simpler terms, it is a translation layer between the analog fax world and the digital IP world.
Traditional fax machines communicate using the T.30 protocol — a sequence of audio-frequency tones sent over the Public Switched Telephone Network (PSTN). That works fine on analog phone lines. But when offices switch to VoIP, those tones have to travel over IP networks that were designed for voice, not fax. The result is frequent failures.
T.38 fixes this by intercepting the fax signal at a gateway and repackaging it into IP-friendly data packets that preserve the fax information without relying on audio fidelity. The receiving gateway converts the data back into T.30 tones for the destination fax machine.
Key facts about T.38:
- Standard: ITU-T T.38 (first published 1998, regularly updated)
- Purpose: Real-time fax relay over IP networks
- Transport: UDP (preferred) or TCP
- Signaling: Uses SIP or H.323 for call setup
- Redundancy: Built-in packet redundancy to handle loss
- Compatibility: Works with all standard Group 3 fax machines
For background on how traditional fax transmission works at the signal level, see our guide to how fax machines work.
How T.38 Works (Step by Step)
The T.38 protocol operates through a relay process. Here is what happens when a fax is sent over an IP network using T.38:
Fax Machine Initiates Call
The sending fax machine dials the recipient's number and begins the T.30 handshake — the familiar sequence of beeps and tones. At this point, the fax machine has no idea it is talking to anything other than a regular phone line.
T.38 Gateway Intercepts the Signal
A T.38-capable gateway (built into the VoIP adapter, PBX, or cloud service) detects the fax tones. Instead of compressing them like voice audio, it decodes the T.30 protocol commands and converts them into T.38 data packets.
Data Travels Over IP
The T.38 packets are sent over the IP network using UDP (preferred for lower latency) or TCP. Each packet contains fax page data plus redundancy information — duplicate copies of previous packets — so the receiving end can reconstruct missing data.
Receiving Gateway Converts Back
A T.38 gateway on the receiving end converts the data packets back into T.30 analog tones and delivers them to the destination fax machine. The receiving machine "thinks" it is communicating directly with another fax machine on a phone line.
Spoofing Hides Network Delays
T.38 gateways use a technique called "spoofing" — they send artificial timing signals to both fax machines to keep the T.30 protocol from timing out while packets traverse the IP network. This is critical because fax machines expect responses within strict millisecond windows.
Both Ends Must Support T.38
T.38 only works when both the sending and receiving gateways support it. If any segment of the network path lacks T.38 support, the fax falls back to audio passthrough — reintroducing all the usual VoIP fax problems like jitter and codec compression.
T.38 vs. G.711 Passthrough
T.38 is not the only way to fax over IP. The other common method is G.711 passthrough — and understanding the difference is crucial for choosing the right approach.
How G.711 Passthrough Works
G.711 is an uncompressed audio codec that preserves the full 64 kbps audio signal. When used for faxing, the VoIP system simply treats the fax tones as uncompressed voice audio and passes them through the IP network as RTP (Real-time Transport Protocol) packets.
Unlike T.38, G.711 does not "understand" that it is carrying fax data. It makes no special accommodations for fax timing, error correction, or packet loss.
Head-to-Head Comparison
| Feature | T.38 Fax Relay | G.711 Passthrough |
|---|---|---|
| How it works | Converts fax to fax-specific data packets | Passes fax tones as uncompressed audio |
| Packet loss tolerance | Tolerates 10%+ with redundancy | Fails at 1% packet loss |
| Bandwidth | Lower (fax data only) | Higher (64 kbps continuous audio stream) |
| Error handling | Built-in redundancy and retransmission | None — relies on network quality |
| Setup complexity | Higher (both ends must support T.38) | Lower (just set codec to G.711) |
| Interoperability | Requires T.38 support on both gateways | Works with any G.711-capable system |
| Reliability | High (designed for fax) | Moderate (depends on network conditions) |
| Cost | Often included in business VoIP plans | Included in any VoIP service |
Which Should You Use?
Choose T.38 if:
- Your network has any packet loss or jitter
- You fax regularly and need high reliability
- Both your provider and the receiving end support T.38
- You send compliance-critical documents (healthcare, legal, financial)
Choose G.711 passthrough if:
- Your network is exceptionally stable with near-zero packet loss
- Your VoIP provider does not support T.38
- You only fax occasionally and can tolerate occasional failures
- You need the simplest possible setup
Choose cloud fax if:
- You don't want to think about protocols at all
- You need 98%+ delivery rates without VoIP configuration
- You fax from phones, computers, or tablets — not just fax machines
Skip the Protocol Debate
mFax bypasses VoIP entirely. It connects directly to the phone network through dedicated fax infrastructure — so T.38 vs. G.711 is someone else's problem. 98% delivery rate, confirmation receipts, no configuration.
When T.38 Matters (And When It Doesn't)
T.38 is important if you are running fax machines on a VoIP phone system. But it is not the only — or even the best — solution in every situation.
You need T.38 when:
- Your office uses VoIP and you still have physical fax machines connected via an ATA (Analog Telephone Adapter)
- You are migrating from PSTN to SIP trunking and need fax to keep working during the transition
- You manage a PBX (like Asterisk, FreePBX, or 3CX) and handle fax calls through your phone system
- You send high-volume faxes through a fax server that connects to the PSTN via VoIP
You don't need T.38 when:
- You use a cloud fax service — the provider handles the protocol layer (cloud fax guide)
- You fax from your phone or computer — apps like mFax manage everything
- You still have a dedicated analog phone line — traditional fax machines work fine on PSTN without any IP protocol
- You've gone fully digital — if both sender and receiver use online fax services, no analog-to-IP conversion happens
For most individuals and small businesses, switching to digital fax eliminates the need for T.38 entirely.
Common T.38 Problems and How to Fix Them
Even when T.38 is enabled, things can go wrong. Here are the most common issues:
Partial T.38 Support
Problem: Your VoIP provider advertises T.38, but only some network segments support it. When the fax traverses a non-T.38 segment, it falls back to audio — and fails.
Fix: Ask your provider specifically: "Is T.38 supported end-to-end on your network, including to PSTN breakout?" If not, T.38 will be unreliable.
SIP Re-INVITE Failures
Problem: T.38 often requires a SIP re-INVITE to switch from voice mode to fax mode mid-call. Some SIP proxies, session border controllers (SBCs), or firewalls block or mishandle this re-INVITE.
Fix: Check your SBC and firewall rules. Ensure SIP re-INVITE is allowed and that T.38 UDPTL (UDP Transport Layer) ports are open and forwarded correctly.
Redundancy Misconfiguration
Problem: T.38 redundancy settings don't match between the two gateways. One side sends redundant packets, the other doesn't expect them — causing decode errors.
Fix: Set redundancy to the same level on both sides. A setting of 1 (one redundant copy of the previous packet) is a good default. Higher values use more bandwidth but tolerate worse networks.
Codec Negotiation Failures
Problem: The call starts as a voice call with a compressed codec (like G.729), and the switch to T.38 fails during negotiation.
Fix: Configure your ATA or PBX to prefer G.711 as the initial codec for fax calls. This gives the cleanest starting point before the T.38 switch. For detailed VoIP fax settings, see our VoIP fax troubleshooting guide.
Quick Test
After adjusting T.38 settings, send a test fax to verify everything works. Test with a single page first, then a multi-page document at different resolutions.
T.38 vs. Cloud Fax: The Bigger Picture
T.38 solves the problem of faxing over IP — but it does so by adding complexity. You need compatible gateways on both ends, proper SIP configuration, open firewall ports, and matching redundancy settings.
Cloud fax takes a different approach: instead of making fax work on your VoIP system, it moves fax off your phone system entirely. The cloud provider maintains dedicated fax infrastructure that connects directly to the PSTN — bypassing VoIP, T.38, and G.711 altogether.
| Factor | T.38 on VoIP | Cloud Fax (mFax) |
|---|---|---|
| Reliability | 85–95% (when configured correctly) | 98% |
| Setup | ATA/PBX configuration, firewall rules, provider support | None — sign up and send |
| Hardware | Fax machine + ATA or fax server | Phone, computer, or tablet |
| Ongoing maintenance | Monitor settings, troubleshoot failures | None |
| Works from mobile | No | Yes |
| Delivery confirmation | Depends on configuration | Automatic |
For organizations that must keep physical fax machines on a VoIP network, T.38 is the best available protocol. For everyone else, cloud fax is simpler, more reliable, and more flexible.
Learn more about how cloud fax works and whether online fax is secure.
Send Your Fax — Without Worrying About Protocols
The T.38 protocol is a smart solution to a real problem: making fax survive on networks that were never designed for it. If you manage VoIP infrastructure and need to support fax machines, understanding T.38 is essential.
But if you just need to send and receive faxes reliably — from your phone, computer, or tablet — you don't need to configure any protocol at all.
mFax.to handles the transmission layer for you. Upload a PDF or snap a photo, enter the fax number, and send. 98% delivery rate with confirmation receipts. No fax machine, no VoIP settings, no T.38 debugging.
For business teams, mFax Business adds virtual fax numbers, team accounts, HIPAA compliance features, and a shared dashboard — starting at about $9/mo (billed annually). There are no rigid fixed tiers: you build your own plan with a live calculator, choosing the exact seats and pages you need and paying only for what you use.