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.
Every SIP Provider Claims T.38 Support. Read That Claim Carefully.
Telnyx supports T.38. VoIP.ms, too. Every provider you’ve evaluated has checked that box, and if your fax keeps failing, you’ve probably been told the problem is your ATA, your baud rate, or your ECM settings. You’ve lowered the speed. You’ve turned off ECM. You’ve tried G.711 fallback. Sometimes it worked. Then it didn’t.
The problem isn’t your equipment. It isn’t your configuration. It’s that “T.38 support” means different things depending on whether the carrier who said it built their network for fax or for voice.
For voice-first providers, T.38 support means pass-through: your fax traffic leaves their network and terminates on an upstream carrier they don’t control, can’t configure, and can’t diagnose when something breaks. The T.38 conversion happens somewhere else, on someone else’s infrastructure. ECM, the part of the protocol that detects corrupted pages and requests retransmission, is typically disabled on those upstream paths to reduce processing load. Routing can change from call to call. When a fax fails, nobody in the chain owns the failure.
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.
What 3CX’s Own Compatibility List Shows
3CX’s certified SIP trunk compatibility page is one of the most referenced resources in the VoIP engineering community. It documents which providers actually work with 3CX deployments and how. For fax specifically, the list is instructive.
Flowroute’s fax support is listed as No. Not limited, not bypass, not requires configuration. No fax support. Telnyx and VoIP.ms are listed as Yes — T38 Bypass. “Bypass” is 3CX’s label for pass-through: the provider relays your fax traffic upstream rather than terminating it on native T.38 infrastructure. Every voice-first provider on that list that carries fax at all carries it the same way.
This isn’t a 3CX problem. It isn’t a configuration problem. It’s what voice-first architecture produces when you ask it to handle a protocol it wasn’t built for.
The people who built T38Fax ran this same comparison at scale, with production fax traffic, across every major SIP provider available, before T38Fax existed. Our parent company, iFAX Solutions, built HylaFAX Enterprise, one of the most widely deployed enterprise fax server platforms in the world. When our own customers needed a SIP trunk we could recommend, we tested every carrier that claimed T.38 support.
Not a single one passed.
So we stopped looking and built one ourselves. In 2013, T38Fax launched as a dedicated fax carrier with a single focus: make T.38 work the way the protocol was designed to work. We ran exhaustive interoperability testing with Dialogic, the company behind the SR140 processing card that runs inside OpenText RightFax, HylaFAX Enterprise, GFI FaxMaker, and XMedius, and achieved the highest interoperability test scores Dialogic had ever recorded.
The Two Things That Determine Whether Fax Actually Works
Consistent T.38
We built our own T.38 stack from the ground up and place our media gateways inside carrier networks. Your fax equipment talks to our gateways on every call — not an upstream carrier we don't control. No bypass. No variable routing. No mystery carrier in the chain. When something fails, we can diagnose it because we own the entire path.
ECM Error Correction
Voice-first carriers disable ECM to reduce upstream processing load — including the carriers handling your pass-through fax traffic. Without ECM, a page can arrive corrupted with neither sender nor recipient knowing the transmission "succeeded." We insist on ECM for every call. The word "facsimile" means exact copy. ECM is how you deliver one.
What Happens When You Move to a Fax-Native Carrier
The pattern from customers who switch from Telnyx, Flowroute, or VoIP.ms is consistent: they spent weeks or months troubleshooting what turned out to be a carrier architecture problem, not an equipment problem, not a configuration problem. One 3CX administrator who had worked through T.38 re-invite issues with Telnyx ended up moving all his fax lines to T38Fax and connecting his ATA devices directly to our network. The fax issues stopped. That outcome isn’t unusual. It’s the point of a fax-native carrier.
If you want to verify how your current SIP trunk handles T.38 and ECM before you switch, run the free ECM self-test at foip.t38fax.com/ecm-self-test. It tells you in a few minutes whether your current provider properly negotiates T.38 with ECM enabled. Most customers who run it find out their provider doesn’t.
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. Consistent T.38, ECM on every call, compliance, and support that diagnoses with protocol traces instead of guesswork.
Why T38Fax →Frequently Asked Questions
- Telnyx supports T.38 as a bypass feature — they relay your fax traffic to an upstream carrier rather than terminating it on their own infrastructure. On 3CX’s certified SIP trunk compatibility list, Telnyx is listed as ‘Yes — T38 Bypass.’ In practice this means fax reliability depends on the upstream carrier chain, ECM behavior is inconsistent, and the configuration troubleshooting specific to Telnyx T.38 re-invites is well-documented in the 3CX community forums. T38Fax runs a native T.38 implementation with ECM enabled on every call — no upstream handoff, no bypass.
- Flowroute does not have fax support listed on 3CX’s certified SIP trunk compatibility page — fax is marked ‘No.’ This reflects the practical outcome of a voice-first architecture: Flowroute’s network was built for voice, and their T.38 handling is inconsistent enough that 3CX does not certify it for fax use. If you are running a 3CX deployment and need reliable fax, the certified list is telling you what technical troubleshooting will confirm: pass-through from a voice-first carrier is not a reliable fax path.
- VoIP.ms is listed on 3CX’s compatibility list as ‘Yes — T38 Bypass,’ which is accurate: they pass your fax traffic upstream rather than terminating it on a native T.38 stack. Users on Asterisk, FreePBX, and similar platforms consistently report intermittent results — fax completing on some calls and failing on others with no clear pattern. That inconsistency is the signature of a pass-through architecture where routing and ECM configuration change from call to call and no single carrier owns the outcome.
- The difference is architectural. Telnyx, Flowroute, and VoIP.ms are voice-first providers — they built their networks for voice calls and added T.38 handling because customers asked for it. T38Fax was built specifically for fax from the start. We run our own T.38 stack, built in partnership with Dialogic — the company whose SR140 processing card is embedded in OpenText RightFax, HylaFAX Enterprise, GFI FaxMaker, and XMedius. We place our media gateways inside carrier networks. ECM runs on every call. There is no upstream carrier in your fax call path and no routing that changes between calls. When a fax fails, we diagnose it with T.30 protocol traces and give you a specific answer — not a suggestion to lower your baud rate.
- ‘T38 Bypass’ is 3CX’s label for pass-through — the SIP provider relays your fax traffic to an upstream carrier rather than handling T.38 on their own infrastructure. The quality of that upstream carrier’s T.38 implementation is outside the provider’s control. ECM is typically disabled on upstream carrier paths to reduce processing load. Routing can differ from call to call depending on which carrier handles a given fax. The result is fax that works on some calls and fails on others, with no actionable diagnostic available because no single party owns the stack end to end.
- Ask them two questions: where does your T.38 conversion happen, and what upstream carrier handles the T.38 leg? If the answer involves ‘relay,’ ‘upstream carrier,’ or ‘our network partner,’ you are looking at pass-through. You can also run T38Fax’s free ECM self-test at foip.t38fax.com/ecm-self-test — it confirms in minutes whether your current SIP trunk properly negotiates T.38 with ECM enabled. Most customers who run it find their current provider fails.
- 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 fax equipment directly at T38Fax without any hardware changes, and the failures they experienced with their previous provider stop. Configuration guides for common setups — Asterisk/FreePBX, 3CX, FreeSWITCH, Cisco, and RightFax/HylaFAX Enterprise — are in our knowledge base.