T.38 is the ITU-T protocol that lets fax travel reliably over IP networks, and open source fax software built on it replaces analog fax machines without losing the reliability or compliance that regulated industries depend on. Platforms like ICTFax use T.38 to send and receive faxes over the internet, so you drop the dedicated lines and hardware while keeping faxes auditable and secure.
Fax is still everywhere in healthcare, legal, finance, and government because it remains a legally accepted, traceable way to move documents. The problem is the old way of doing it: analog lines, physical machines, toner, and no flexibility. T.38 over IP solves that, and open source gives you the software to run it on standard servers. Here’s how the protocol works, which platforms support it, and how to choose.
What is T.38?
T.38 is a protocol from the International Telecommunication Union (ITU-T) that carries fax over IP, often called Fax over IP or FoIP, in real time. It was designed specifically to solve the problem of sending faxes across packet-switched networks like the internet, where the timing-sensitive analog fax handshake would otherwise break.
The mechanics matter because they explain why T.38 is more reliable than just streaming fax audio over a voice codec:
- It converts the analog fax signal into digital packets at the gateway.
- It transports those packets over UDP, typically using UDPTL, with RTP as an alternative.
- It adds error correction so a dropped packet doesn’t corrupt the whole page.
- It keeps latency low enough that the fax machines on each end stay in sync.
That’s why T.38 is the standard choice in SIP-based VoIP systems: it preserves document integrity even when network conditions aren’t perfect. The alternative, G.711 pass-through, simply carries the fax tones as audio and is far more fragile on a busy network.
Why use open source fax software with T.38?
Building your faxing on a T.38 open source platform gives you a cost-effective, scalable alternative to hardware fax machines. These platforms run on standard servers and support the workflows people actually use: email to fax, fax to email, web to fax, and API-driven sending. You can read more on the FoIP approach and ATA-device options in our guide to FoIP faxing and fax machine ATA devices.
Open source platforms that support T.38
Four projects cover most real-world T.38 deployments. Here’s where each fits.
| Platform | What it is | T.38 role | Best for |
|---|---|---|---|
| ICTFax | Full FoIP server (FreeSWITCH + ICTCore + Angular) | T.38 and G.711, send and receive | Turnkey email/web/API faxing, multi-tenant |
| FreeSWITCH | Telephony platform | Native T.38 gateway | Custom fax builds, high availability |
| Asterisk | PBX system | T.38 pass-through and gateway (via modules) | Fax inside a unified comms setup |
| HylaFAX+ | Dedicated fax server | T.38 with compatible modems | High-volume Unix/Linux fax queues |
ICTFax
ICTFax is a feature-rich open source FoIP solution built on top of FreeSWITCH, ICTCore, and Angular. It supports both T.38 and G.711 pass-through faxing, so businesses send and receive faxes over the internet without traditional phone lines. Its feature set covers email to fax and fax to email, a web-based dashboard, multi-tenant user portals, PDF/TIFF/JPG support, and a REST API for integration. Because the heavy lifting runs on FreeSWITCH rather than Asterisk, it scales cleanly for service providers and enterprises. The SIP fax server overview goes deeper on the architecture.
FreeSWITCH
FreeSWITCH is a scalable open source telephony platform that supports T.38 faxing natively, which makes it a strong base for custom fax solutions. It integrates well with modern SIP infrastructure, is scriptable in Lua, Python, and JavaScript, and handles high-availability deployments. ICTFax and ICTCore build directly on it.
Asterisk
Asterisk is an open source PBX that supports T.38 faxing through compatible endpoints or modules such as Fax For Asterisk. It fits best when you want fax to live alongside voice and video in a single unified communications setup, with the flexible dialplan handling routing.
HylaFAX+
HylaFAX+ is a long-established open source fax server for Unix and Linux that supports several transmission protocols including T.38. It’s built for high-volume faxing, with strong scheduling, queue management, email integration, and command-line tooling for scripted workflows.
Benefits of T.38-based open source fax software
- Cost efficiency: no fax machines, dedicated lines, toner, or paper, which cuts both capital and operating costs.
- Remote access: send and receive faxes from anywhere through web portals or email, which suits remote and hybrid teams.
- Scalability: from a small business to a large service provider, you can add capacity and handle hundreds or thousands of concurrent faxes.
- Security and compliance: support for TLS, HTTPS, and authenticated SMTP helps meet standards like HIPAA, GDPR, and PCI DSS.
- Interoperability: these platforms integrate with VoIP systems, CRMs, and document management tools through APIs.
If you want to try it, the ICTFax download and the fax API tutorial are the fastest way to get a working T.38 fax server running.
Frequently asked questions
What is the difference between T.38 and G.711 faxing?
T.38 converts the fax into digital packets with error correction, which makes it resilient on imperfect networks. G.711 pass-through simply carries the fax tones as audio, so it’s simpler but far more prone to failure when there’s jitter or packet loss. For reliable FoIP, T.38 is the preferred choice.
Is open source fax software HIPAA compliant?
The software can support HIPAA compliance through TLS, HTTPS, and authenticated SMTP, but compliance is ultimately about how you deploy and operate it. The platform gives you the encryption and access controls; your policies, retention rules, and documentation complete the picture.
Does ICTFax run on Asterisk or FreeSWITCH?
ICTFax is built on FreeSWITCH, together with ICTCore and an Angular front end. FreeSWITCH handles the T.38 and G.711 media and SIP signaling, which is what gives ICTFax its scalability for multi-tenant and service-provider use.
Can I send faxes by email with T.38 software?
Yes. Email to fax and fax to email are standard features in platforms like ICTFax. You send a document as an email attachment and the server transmits it as a fax over T.38, and incoming faxes arrive in your inbox as PDF or TIFF files.
Do I need special hardware for T.38 faxing?
No dedicated fax machine is required. T.38 runs on standard servers over your SIP trunk or VoIP provider. You only need analog hardware, such as an ATA, if you want to connect a legacy fax machine into the IP-based setup.
Conclusion
T.38-based open source fax software is the modern replacement for outdated fax machines. ICTFax, FreeSWITCH, Asterisk, and HylaFAX+ cut operating costs while bringing flexibility, reliability, and security to fax. If you want to digitize your fax infrastructure, ICTFax is a strong T.38-compliant starting point that handles email, web, and API faxing while supporting compliance. Need help getting started? Open a ticket and the team will help you deploy and configure your T.38 fax server.