Why T38Fax
The Fax Carrier That Actually Understands Fax
T38Fax was built from the ground up for one purpose: reliable T.38 fax over IP at scale. Here's what that means — and why it matters to your operation.
Built for Fax. Not Adapted for It.
Most companies that offer T.38 are voice carriers who added fax as an afterthought. Their infrastructure was designed for voice calls — predictable, tolerant of modest packet loss, forgiving of timing imprecision. Fax is none of those things. T.38 requires exact timing, clean UDPTL handling, and Error Correction Mode on every call. Voice networks weren’t built for it.
T38Fax was. We’re a company of career fax industry professionals who spent years watching voice carriers fail at fax — and built an alternative. Every technical decision we’ve made since day one has been optimized for one thing: reliable fax transmission at scale.
The four things that actually make the difference are below.
Fax Over IP, Explained
Before getting to what makes T38Fax different, it is worth grounding the conversation in what fax over IP actually is — because most of the industry conversation is muddled by carriers calling things “T.38 support” that are not, and “fax-friendly” implementations that are not.
The Real-Time Standard for Fax Over IP
T.38 is the ITU standard for transmitting fax in real time over IP networks. When your fax machine, fax server, or ATA sends a fax, T.38 converts the fax tones into a data protocol called UDPTL and carries them across the network — the same way the traditional phone network carries fax tones as audio. The fax arrives the moment it is sent. There is no upload, no cloud storage, no portal to log into.
This is what distinguishes T.38 from the cloud fax and “fax as email attachment” services that came later. Those services convert your document into a different format, store it on the provider’s servers, and deliver it through a separate transmission. T.38 is the protocol; cloud fax is a product layered on top of telecommunications. The two solve different problems.
T.38 and G.711 Fax Are Not the Same Thing
Most SIP carriers will tell you they support fax. What they typically mean is G.711 passthrough — encoding fax tones as audio packets and sending them like voice. G.711 works on short, clean network paths but degrades quickly with any packet loss, jitter, or re-routing. The packet loss numbers that pass unnoticed on a voice call corrupt fax transmissions silently.
T.38 was designed specifically for the realities of IP networks. UDPTL includes built-in redundancy that allows fax data to survive the packet loss that G.711 cannot. A real T.38 implementation does not lose pages to network conditions that a voice call would barely notice. When a carrier offers “fax support” without specifying T.38 — or worse, offers G.711 passthrough and calls it T.38 — it produces the inconsistent fax performance that fills helpdesk tickets at every IT department running fax on a VoIP network.
Error Correction Mode Is What Makes a Fax Match Its Original
A facsimile is meant to be an exact copy. Error Correction Mode (ECM) is the part of the T.30 fax protocol that makes that promise enforceable: it divides each page into frames, validates each frame against a checksum, and requests retransmission of any frame that arrives corrupted. The standard has been in fax machines since 1991. When ECM is enabled end-to-end and the carrier respects it, a fax that completes transmission is byte-for-byte identical to what was sent.
What goes wrong in practice is that ECM gets disabled — often by the carrier, sometimes by the equipment vendor, occasionally by both — because turning it off reduces processing load. Without ECM, the protocol still completes calls and prints confirmation pages, so the failure mode is silent. Most “the fax went but the recipient did not get it” tickets are ECM failures.
Fax Over IP Is Reliable — When the Implementation Is Right
The reputation that fax over IP is unreliable comes from the lived experience of running fax on networks that were not built for it. It is not a property of the protocol. T.38 is a mature standard, ECM has been in fax machines since the early 1990s, and the engineering required to make fax over IP work as reliably as a POTS line is well-understood. The reason most VoIP carriers struggle with fax is not that fax over IP is hard — it is that the carriers are voice companies for whom fax is a footnote.
Consistent T.38
Most SIP providers don't actually run their own T.38 infrastructure — they pass your fax traffic to an upstream carrier whose implementation they don't control and can't fix when it breaks. When you file a support ticket, they escalate to a carrier who escalates to another carrier, and the answer comes back as "try lowering your fax speed."
T38Fax built its own T.38 stack from the ground up. We place our media gateways inside carrier networks and terminate G.711 audio directly to the PSTN over a single optical hop. Your equipment talks to our gateways on every call. No upstream handoff. No variable routing that changes call by call. No mystery carrier in the chain whose behavior we can't control.
Read MoreECM Error Correction
Error Correction Mode (ECM) is the part of the fax protocol that detects corrupted pages and requests retransmission. Without it, a page can arrive garbled — or half-blank, or missing its last three lines — with neither the sender nor recipient knowing it happened. The transmission "succeeds." The confirmation page prints. The content is wrong.
Most carriers disable ECM to reduce processing load. It's a quiet decision, invisible to their customers, that makes their fax statistics look fine while your users wonder why critical documents keep arriving unreadable. We insist on ECM for every call. The word "facsimile" means exact copy. That's not optional.
Why It MattersSecurity & Compliance
T38Fax is SOC 2 certified and HIPAA compliant. Because T.38 is a real-time protocol, fax content passes through our network and terminates — we never store it on our servers. This simplifies your compliance posture considerably compared to store-and-forward fax services, where transmitted documents sit in a database that has to be secured, audited, and defended.
For organizations that require encrypted transport, we offer optional IPSec VPN tunnels at no additional charge. Private dedicated circuits and direct cloud connections into AWS and other hosting environments are available for stricter requirements.
Security & Compliance DetailsPremium Support
We don't just support the product — we genuinely believe support is the product. Our team consists of fax industry veterans who diagnose issues with protocol traces and T.30 analysis. When you open a ticket, you reach someone who understands the difference between a T.38 negotiation failure, an ECM partial-page retransmission issue, and a misconfigured ATA. You get a real diagnosis, not a suggestion to lower your fax speed and try again.
Unlimited support is included on every plan — free trial through enterprise. We don't gate technical support behind a paid tier. If you're having a problem with your fax configuration, we want to fix it.
Visit the Knowledge BaseWhat Our Customers Say
"We are a VoIP provider with a sizable municipal and government customer base. Trying other carrier's fax over VoIP services proved to be disastrous. For years our policy was POTS ONLY for fax, that is until we discovered T38Fax. After initial testing, we decided to start rolling out to our customers. The support from Stefanie, Morgan, and the whole team has been flawless. I can safely say that they are my favorite carrier to work with!"
— Joseph Sauer, Project Manager, DBO-TSG"T38Fax's customer service and speedy response is second to none. They don't just send you a canned company answer that further confuses you like other companies do. They are super friendly and genuinely want to help."
— Steve Kile, Senior Systems Admin, Yosemite Farm CreditRead more from the organizations that depend on T38Fax every day.
Read All TestimonialsSee the full Power-T.38 product page — architecture details, interoperability certifications, and deployment options.
Our Solution: Power-T.38Frequently Asked Questions
- T.38 is the ITU standard protocol for transmitting fax documents over IP networks in real time. Unlike sending a fax as an email attachment or storing it in the cloud, T.38 carries live fax signals — the same way the traditional phone network does — but over SIP instead of POTS. It handles the timing, error correction, and signal conversion that make a fax look exactly like what was sent. When it’s implemented well, fax over IP is indistinguishable from a POTS line. When it’s implemented poorly — which describes most SIP providers — pages fail, calls drop, and nobody can tell you why.
- Most SIP providers don’t run their own T.38 infrastructure. They pass your fax traffic to an upstream carrier whose implementation they don’t control. When something goes wrong, they can’t fix it — because it’s not their system. T38Fax built its own T.38 stack. We place our media gateways inside carrier networks and terminate G.711 audio directly to the PSTN over a single optical hop. Your equipment talks to our gateways on every call. There’s no upstream handoff, no variable routing, and no mystery carrier in the chain whose behavior changes from call to call.
- If it speaks T.38 FoIP, it will work with us. Our Compatibility List covers everything we’ve tested ourselves — fax servers (RightFax, HylaFAX Enterprise, GFI FaxMaker, XMedius), softswitches and IP-PBXs (FreePBX, Asterisk, 3CX, Cisco BroadWorks, FreeSWITCH, NetSapiens), ATA gateways (Grandstream, Cisco SPA, Patton), and a selection of desktop fax machines with native T.38 support. For devices not on the list, our support team is happy to work through the configuration with you — bring us your device and we’ll figure it out.
- Yes. T38Fax is HIPAA compliant and SOC 2 certified. Because T.38 is a real-time protocol, fax content passes through our network and terminates — we never store it on our servers. This simplifies your compliance posture considerably compared to store-and-forward fax services, where documents sit in a database someone has to secure and audit. If your organization requires encrypted transport, we offer optional IPSec VPN tunnels at no additional charge. Private dedicated circuits and direct cloud connections (AWS and others) are available for environments with stricter requirements. A Business Associate Agreement is available on request.
- Security at T38Fax operates at multiple layers. At the network level, we offer IPSec VPN tunnels to encrypt the SIP signaling and media path between your infrastructure and ours. At the compliance level, we are SOC 2 certified and HIPAA compliant — our Security & Compliance page covers our full posture in detail. At the protocol level, the real-time nature of T.38 means we never store fax content — there’s no database of your transmitted documents on our servers. For organizations requiring physical network separation, we can provide dedicated private circuits.
- We offer simple, usage-based pricing with no activation fees, no minimum term, and no volume commitments required to get started. Unlimited number porting is included at no charge. Volume discounts are available for usage above 1,000 minutes per month. Wholesale pricing is available for service providers and resellers who can meet volume thresholds. See our Pricing page for full details, or contact our sales team with questions about your specific volume.
- Sign up for a 30-day free trial. The trial includes full access to the service and unlimited technical support — our team genuinely enjoys a configuration challenge, so bring whatever you’re working with. When the trial ends, the account stops working unless you activate it to a paid plan. There’s no automatic conversion.
T.38 Explained
T.38 Is Key to Faxing Reliably Over the Internet
Not all fax-over-IP services use T.38 — and the ones that don't make it obvious when faxes start failing. This guide explains how T.38 works, why ECM matters, and what to look for in a carrier that takes reliable fax transmission seriously.
Read the Guide