Power-T.38
The Fax-Native SIP Trunk
Power-T.38 is T38Fax's flagship SIP trunking service, engineered specifically for fax. In-house T.38 stack. ECM always enabled. Collocated inside carrier networks. One product, deployed three ways.
Power-T.38 Is a Single SIP Trunking Service
Power-T.38 is T38Fax’s flagship fax-optimized SIP trunking service. The product is singular—one T.38 stack, one network, one set of engineering decisions that make it work. The reasons to use it vary by how your organization sends and receives fax.
Enterprise Fax Servers
Running RightFax, HylaFAX Enterprise, GFI FaxMaker, or XMedius? Power-T.38 was engineered around the Dialogic SR140—the same T.38 engine family your fax server uses. Deep interoperability, not coincidental compatibility.
Enterprise Fax Servers →Voice Service Providers
Tired of fielding fax support tickets you can't actually fix? Offload fax traffic to our network. Port your customers' fax DIDs to T38Fax. You keep the customer relationship; we handle the T.38.
Voice Service Providers →Business Fax Connectivity
IP-PBX, ATA, MFP, or a mix of all three—Power-T.38 connects to virtually any T.38-capable device. If your VoIP migration broke fax, this is the fix.
Business Fax Connectivity →The Engineering Decisions That Make the Difference
Most T.38 failures trace back to the same source: a voice-centric carrier whose T.38 “support” is really T.38 pass-through to an upstream provider they don’t control. The implementation is inconsistent, ECM is disabled, and when something breaks, nobody can diagnose it because nobody actually owns the stack.
Power-T.38 was built from the premise that fax over IP requires fax expertise—not voice expertise with a fax checkbox.
In-House T.38 Stack
We don't resell our carriers' T.38. We built our own implementation, in partnership with Dialogic, around the SR140 engine—the same T.38 technology embedded in many of the world's most widely deployed fax servers. We own the stack, which means we can diagnose it, modify it, and stand behind it. When something goes wrong, we know exactly where to look.
Collocated Inside Carrier Networks
Our media gateways are physically located inside carrier facilities. T.38 conversion and PSTN hand-off happen within the same facility over a single optical hop—no public internet crossing between them. Your equipment talks to our gateways on every call. There is no upstream carrier handling T.38 on our behalf. This is what "no variable routing" actually means: not a marketing claim, but a physical architecture.
ECM Mandatory on Every Call
Error Correction Mode divides each fax page into frames, verifies each frame, and retransmits any that arrive corrupted. Without ECM, pages can arrive with errors that neither side detects. Carriers disable ECM by default because it adds processing load. We've made the opposite choice: ECM is always on, no exceptions, because a fax that might be wrong is not a fax. "Facsimile" means exact copy—and we intend to deliver one.
Consistent Routing on Every Call
Variable routing is the hidden cause of most intermittent fax failures. When a carrier's routing changes between calls—different upstream paths, different T.38 handling, different ECM behavior—fax reliability becomes unpredictable in ways that are nearly impossible to diagnose. Because your traffic always terminates on our gateways, routing doesn't vary. The same call sent twice takes the same path and gets the same handling. Consistent results from consistent architecture.
Certified for the Equipment You Actually Use
Power-T.38 was built around the Dialogic SR140 T.38 stack—achieving the highest interoperability scores Dialogic had ever recorded. In practice, this means compatibility runs deep with the fax servers, softswitches, and devices that embed the same technology.
Fax Server Software
- OpenText RightFax
- HylaFAX Enterprise
- GFI FaxMaker
- XMedius
- Biscom
- Concord Fax
- Esker
- Castelle
- Softlinx ReplixFax
Softswitches & IP-PBXs
- Asterisk / FreePBX
- FreeSWITCH / FusionPBX
- 3CX
- Cisco CUCM
- Cisco BroadWorks
- Avaya
- NetSapiens
- Metaswitch
- Grandstream UCM Series
ATAs & Gateways
- Grandstream HT801, HT802, HT812, HT814, HT818
- Grandstream GXW4004, GXW4008
- Cisco ATA 191-MPP, ATA 192-MPP
- AudioCodes MediaPack 1xx
- Patton SmartNode
- OBiHAI OBi200, OBi202, OBi504vs, OBi508vs
The full certified device list—with tested configurations and setup guides—is maintained in the Knowledge Base. If your device isn’t listed, bring it to our 30-day free trial. Our support team has seen most of it—and if they haven’t, they want to.
What Our Customers Say
"We've sent several million faxes since migrating to T38Fax, and we've not had a single demonstrable case where faxes fail when using T38Fax but succeed with our previous carrier. Thank you for the outstanding service!"
— GreenFax"We have been using T38Fax for 18 months now. We have six locations in three western states, all 24hr healthcare facilities with critical needs in terms of faxing. After several years, we eliminated virtually every problem we had by deploying T38Fax. These guys have a powerhouse solution, rock solid, always up, that plays wonderfully with our PBX. Our faxes now are more stable than they were with POTS lines."
— Tyson Frantz, Owner, Guardian Angel HomesFrequently Asked Questions
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T.38 is an ITU standard that defines how fax signals are transmitted over IP networks. Unlike voice-over-IP, which encodes audio as compressed packets, T.38 converts fax tones into a purpose-built data protocol called UDPTL. UDPTL includes built-in redundancy that makes it resilient to the packet loss and jitter that cause G.711 fax passthrough to fail. When implemented correctly—with ECM error correction enabled and consistent routing—T.38 fax over IP is as reliable as the traditional phone network.
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G.711 fax over SIP encodes analog fax tones as audio and transmits them as voice packets. It works on short, clean network paths, but degrades quickly with any packet loss, jitter, or re-routing—all of which are normal on real-world WAN connections. T.38 converts fax tones into a dedicated data protocol with built-in error redundancy, making it far more resilient. Most SIP providers offer G.711 passthrough and call it ‘T.38 support.’ It isn’t. Power-T.38 is a native T.38 implementation with ECM error correction always enabled.
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ECM is a feature of the T.30 fax protocol that divides each transmitted page into frames, checks each frame for transmission errors, and requests retransmission of any frame that arrives corrupted. Without ECM, a fax page can arrive with missing or garbled lines—and neither side knows. The word ‘facsimile’ means exact copy. Without ECM, you’re not sending facsimiles; you’re sending approximations. Most SIP carriers disable ECM by default to reduce processing load. T38Fax enables ECM on every call, with no exceptions.
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It means our media gateways—the equipment that handles T.38 conversion—are physically located inside carrier facilities, not in a separate data center connected over the public internet. When your fax equipment sends a T.38 call, it terminates on our gateways. We then convert to G.711 audio and hand it off to the PSTN over a single optical hop, entirely within the carrier facility. There is no public internet crossing between T.38 conversion and PSTN delivery. This is what makes our calls consistent: there is no upstream carrier handling T.38 on our behalf, and no additional network segment to introduce variability.
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The Dialogic SR140 is a software-based T.38 fax processing engine developed by Dialogic—one of the most widely deployed T.38 implementations in the world, embedded in fax server products like OpenText RightFax, HylaFAX Enterprise, GFI FaxMaker, and others. T38Fax partnered with Dialogic to build our network around the SR140 stack, ensuring deep interoperability with the fax servers our customers actually use. We achieved the highest interoperability test scores Dialogic had ever recorded. When your RightFax or HylaFAX installation connects to Power-T.38, it’s communicating with the same T.38 engine family it was designed to work with.
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T.38 pass-through means a provider’s system relays T.38 traffic between your equipment and their upstream carrier—the carrier’s T.38 implementation determines call quality, and the provider has no control over it. With Power-T.38, your T.38 traffic terminates on our network. We run our own T.38 stack, built around the Dialogic SR140, and handle conversion ourselves. The upstream carrier never touches your T.38 traffic. This distinction is the entire reason T38Fax exists: we couldn’t find a provider whose T.38 pass-through was reliable enough, so we built a network where pass-through doesn’t exist.