Thoroughly tested to ensure T38 compatibility and eliminate many of the issues found with fax over SIP.
Premium fax SIP trunk is here. We've created a custom outbound profile specifically designed for completing faxes over SIP. This outbound profile has been thoroughly tested to ensure T.38 compatibility and eliminate many of the issues found with fax over SIP. This is a premium fax trunk, and you'll notice that there is a separate fax rate deck that can be downloaded from the pricing page.
While some may consider fax outdated, it remains critical for industries requiring secure, compliant document transmission. According to , approximately 70% of healthcare organizations still rely on fax for external communications. The reports that mail and fax remain the primary methods hospitals use to send and receive health information.
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"The fax services market is expected to reach USD 6.5 billion by 2029, rising at a CAGR of 11.08%. Healthcare providers frequently exchange a high volume of sensitive documents, including medical records, lab results, prescriptions, referral letters, billing information, and insurance claims. This continuous need for secure and reliable communication sustains the demand for fax services."
— Research and Markets, Fax Services Market – Focused Insights 2024-2029, ResearchAndMarkets.com
Businesses in healthcare, finance, and legal sectors continue to depend on fax because of its point-to-point transmission security and compliance advantages. For organizations handling sensitive data, our Fax API enables modern, cloud-based faxing without legacy hardware.
T.38 is an ITU-T recommendation for transmitting faxes across IP networks in real time. As Wikipedia explains, the T.38 fax relay standard was devised in 1998 to transport faxes across IP networks between existing Group 3 fax terminals, bridging traditional fax with modern VoIP infrastructure.
Traditional voice codecs compress audio in ways that cause packet loss and jitter, which fax transmissions cannot tolerate. T.38 addresses this by converting fax signals into data packets specifically optimized for IP transmission. According to a Dialogic white paper on T.38 vs. G.711, T.38 provides superior reliability across varying network conditions and is the preferred protocol for mission-critical fax delivery.
To use premium fax SIP trunk, click on the Outbound menu option in your Mission Control Portal:
And you should be good to go.
For detailed configuration steps, refer to our SIP Trunking Getting Started guide in the developer documentation.
By default, Telnyx will send a T.38 re-INVITE message once FAX tone is detected. You can customize this behavior through your connection settings. Our support article on Fax service with Telnyx via T.38 or G711 provides detailed configuration instructions for both inbound and outbound fax.
For advanced configuration options including T.38 re-INVITE control, refer to our guide on SIP Connection: Inbound & Outbound Settings.
Understanding your fax transmission options helps you choose the right configuration for your use case:
| Feature | T.38 Protocol | G.711 Pass-Through | Traditional PSTN Fax |
|---|---|---|---|
| Transmission method | Data packets over IP | Audio encoding over IP | Analog signal over copper |
| Bandwidth efficiency | High (optimized packets) | Low (64 kbps uncompressed) | N/A |
| Packet loss tolerance | High (built-in redundancy) | Low (no error correction) | N/A |
| Error correction | T.38 native ECM | Dependent on endpoint | Native ECM |
| HIPAA suitability | Excellent (encrypted, real-time) | Good with proper config | Good (point-to-point) |
| Best for | Enterprise FoIP, healthcare | Legacy system compatibility | Traditional offices |
| Telnyx support | Full support | Full support | Via SIP gateway |
| Aspect | SIP Trunking | VoIP Services | Cloud Fax Service |
|---|---|---|---|
| Primary use | Voice, video, messaging, fax | Voice calls | Document transmission |
| Protocol | SIP (Session Initiation Protocol) | Various (SIP, H.323, proprietary) | T.38 or G.711 over SIP |
| Infrastructure | Connects PBX to PSTN via IP | End-user phone service | API or portal-based |
| Scalability | Unlimited channels | Per-user licensing | Per-page or unlimited |
| Hardware required | IP-PBX or SBC | IP phones or softphones | None (cloud-based) |
| Fax capability | Native with fax-certified profile | Often requires gateway | Native |
| Best for | Enterprises with existing PBX | SMBs, remote workers | High-volume document workflows |
| Telnyx solution | Elastic SIP Trunking | Voice API | Fax API |
Learn more about these distinctions in our comprehensive guide: SIP vs VoIP: Breaking down the key differences.
Sending fax traffic over a conversational profile? Not a problem. We'll still accept fax traffic sent using a conversational outbound profile and will complete these faxes as a best effort service. However, if completing your faxes reliably is important, we recommend using the dedicated fax outbound profile.
For organizations building fax into their applications, our Programmable Fax API documentation provides everything you need to send, receive, and manage faxes programmatically.
Healthcare organizations face unique compliance requirements that make secure faxing essential. Our article on why hospitals still use fax explains how regulatory requirements like HIPAA, combined with interoperability challenges between EHR systems, keep fax as the preferred communication method for protected health information.
Key benefits of T.38 fax over SIP for healthcare:
Learn more about leveraging Fax APIs for workflow automation to modernize document handling while maintaining compliance.
Telnyx fax SIP trunking serves organizations across multiple sectors:
Set up your fax SIP trunk today through Mission Control.
For a complete understanding of SIP trunking fundamentals, read our guide: What is SIP Trunking? Session Initiation Protocol Explained.
If you have any questions, please don't hesitate to contact us. Our support team is available 24/7 in Mission Control.
Related resources:
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