Fax-Native vs. Voice-First
Telnyx, Flowroute, and VoIP.ms Support T.38. T38Fax Was Built for It.
Telnyx, Flowroute, and VoIP.ms offer T.38 support. T38Fax is built around it. Here's what that architecture difference means for every fax you send.
“T.38 Support” Doesn’t Mean the Same Thing to Every Carrier
Telnyx lists T.38 support. So do VoIP.ms and plenty of others. If your fax keeps failing, you have probably been told the problem is your ATA, your baud rate, or your ECM settings. You lowered the speed. You turned off ECM. You tried G.711 fallback. Sometimes it worked, then it didn’t.
The problem usually isn’t your equipment or your configuration. It is that “T.38 support” describes very different things depending on whether the carrier who listed it built their network for fax or for voice.
On a voice-first network, fax is a secondary capability. Some of these carriers pass your T.38 traffic through to an upstream carrier they don’t control, so when a page drops, no one in the chain owns the failure from end to end. Others convert T.38 on a media gateway of their own. They own the conversion, but the gateway was built and tuned for voice calls, and fax features such as ECM are often left out.
You have probably been told to:
- Lower your fax baud rate to 9600 or below
- Disable ECM on your ATA or fax server
- Switch from T.38 mode to G.711 pass-through
- Try a different DID or route
None of these fix the carrier. They work around it, and each one makes your fax infrastructure less reliable than the protocol is capable of.
Two Ways to Handle T.38, and Only One Was Built for It
Every SIP provider that lists T.38 handles it one of two ways. Some relay your fax to an upstream carrier they don’t operate. That is pass-through, and it means the provider selling you the trunk can’t fully diagnose a failed fax, because the conversion happens on someone else’s network. Others convert T.38 on a gateway they operate. They own the conversion, but the gateway was built for voice traffic first, and fax features such as ECM are often left out.
That is a fact about how the network is built, not a rating anyone assigns to it.
Before T38Fax existed, its parent company, iFAX Solutions, the maker of HylaFAX Enterprise, needed a SIP trunk it could recommend to its own customers. It tested the SIP providers available at the time that claimed T.38 support, using production fax traffic.
Not a single one passed.
So iFAX built its own. T38Fax launched in 2013 as a dedicated fax carrier with one job: make T.38 work the way the protocol was designed to. We run our T.38 stack around the Dialogic SR140, the engine embedded in RightFax, HylaFAX Enterprise, GFI FaxMaker, and XMedius, and we are one of a small number of SIP trunk providers Dialogic has certified for use with it.
Four Things a Fax Carrier Has to Get Right
Consistent T.38
We built our own T.38 stack and place our media gateways inside carrier networks. On every call, your fax equipment talks to our gateways, not to an upstream carrier we don't control. No pass-through. No variable routing. No mystery carrier in the chain. When a fax fails, we can diagnose it, because we own the path it took.
ECM Error Correction
Error Correction Mode (ECM) is the part of the protocol that catches a corrupted page and asks for it again. It is negotiated at the start of each call, and once established it stays on, but only if the carrier supports it. Many voice-first carriers don't, or don't consistently. Lumen's T.38 doesn't offer ECM at all, and on Telnyx and Flowroute it depends on the path a call takes. Without ECM, a page can arrive damaged with neither side being alerted to the possible distortion. We run ECM on every call.
The word "facsimile" means exact copy. ECM is how you deliver one.
Security & Compliance
T38Fax is SOC 2 certified and HIPAA compliant. Because T.38 is a real-time protocol, fax content passes through our network without being stored on our servers, so there is no archive of your documents to secure or audit. Optional IPSec VPN tunnels encrypt the path between your equipment and ours at no additional charge.
Fax-First Support
Call a voice-first carrier about a failed fax and you're in the same queue as everyone with a dropped call, talking to someone who handles a fax ticket every few months. Call T38Fax and the person who answers is pulling T.30 protocol traces before you've finished describing the problem, because fax is the only kind of ticket we take. That's the difference between an answer today and the multi-day escalation chain a national carrier puts you through when your problem needs someone who actually understands the protocol.
What Happens When You Move to a Fax-Native Carrier
Customers who come to us from Telnyx, Flowroute, VoIP.ms, and other SIP providers tell a similar story. On their previous trunk, they spent weeks or months troubleshooting fax: ATA settings, baud rates, ECM toggles. Once they moved to T38Fax, it became clear the problem had been how their carrier handled fax, not their equipment or configuration.
To check how your own trunk handles T.38 and ECM before you switch, run the free ECM self-test at foip.t38fax.com/ecm-self-test. It shows in a few minutes whether your current provider negotiates T.38 with ECM enabled.
How our T.38 stack is architected. What we built with Dialogic, what ECM does at the frame level, and why gateway placement inside carrier networks matters more than the protocol name on a feature list.
SIP Trunk for Fax →The four reasons customers stay. Details on our architecture, compliance posture, and support team.
Why T38Fax →Frequently Asked Questions
- Telnyx offers T.38, but it runs as a pass-through: your fax traffic is relayed to an upstream carrier Telnyx doesn’t operate, so the outcome of any given fax depends on that upstream path rather than on Telnyx. ECM, the error correction that keeps pages intact, comes and goes on Telnyx depending on the route a call takes. T38Fax terminates T.38 on our own gateways with ECM on every call, and no upstream carrier in your fax path.
- Flowroute is a voice and messaging carrier, and fax isn’t what its network was built for. T.38 over Flowroute is inconsistent, and ECM handling varies with the path a call takes. Like other voice-first providers, it carries fax as a secondary capability rather than terminating T.38 on fax-tuned infrastructure of its own. If reliable fax is the requirement, a carrier built specifically for it is the safer path.
- You can, but VoIP.ms handles T.38 the way most voice-first trunks do: as a pass-through, relaying your fax to an upstream carrier it doesn’t operate. ECM and routing on that upstream leg sit with a carrier VoIP.ms doesn’t control, which is why results vary from call to call. To see how your own trunk behaves, run the free ECM self-test at foip.t38fax.com/ecm-self-test.
- The difference is architectural. Telnyx, Flowroute, and VoIP.ms built their networks for voice and added T.38 handling because customers asked for it. T38Fax was built for fax from the start. We run our own T.38 stack around the Dialogic SR140, the same T.38 engine embedded in OpenText RightFax, HylaFAX Enterprise, GFI FaxMaker, and XMedius, and we are one of a small number of SIP trunk providers certified by Dialogic for use with it. We place our media gateways inside carrier networks, run ECM on every call, and keep any upstream carrier out of your fax path. When a fax fails, we diagnose it with T.30 protocol traces and give you a specific answer.
- Pass-through means your SIP provider doesn’t convert and terminate T.38 itself. It relays your fax to an upstream carrier that does, on infrastructure your provider doesn’t operate and can’t fully diagnose. ECM handling on that upstream leg is out of your provider’s hands, routing can change from call to call, and when a page drops, no single party owns the failure. That is why fax on a pass-through trunk tends to work on some calls and fail on others with no obvious pattern. T38Fax converts and terminates T.38 on its own gateways, so one party is accountable for the whole path.
- Ask your provider two questions: where does the T.38 conversion happen, and which upstream carrier handles the T.38 leg? Answers that mention a relay, an upstream carrier, or a network partner point to pass-through. You can also run the free ECM self-test at foip.t38fax.com/ecm-self-test, which shows in a few minutes whether your current trunk negotiates T.38 with ECM enabled. Many who run it find it does not.
- You reach a fax engineer, not a general queue. The person who answers starts at the fax layer, pulling T.30 protocol traces and looking at ECM negotiation and page-level results, because fax is the only kind of ticket we take. Most issues get a specific diagnosis on the first call, instead of the multi-day escalation a general carrier runs you through before anyone who understands T.38 looks at the problem.
- Yes. T38Fax uses standard SIP trunking, the same connection method your fax equipment, ATA, or fax server already uses with your current provider. Most customers point their equipment directly at T38Fax with no hardware changes. Configuration guides for common setups, including Asterisk/FreePBX, 3CX, FreeSWITCH, Cisco, and RightFax or HylaFAX Enterprise, are in our knowledge base.