Seeking Resolution: T.38 Support for SIP Faxing

Hello VitalPBX Community and Support Team,

I wanted to bring to light an ongoing concern our organization has faced regarding the support of T.38 for SIP faxing with VitalPBX (Outbound faxes).

As many of you might know, T.38 is fundamental for dependable fax transmissions over IP networks. Our business is heavily dependent on this feature. Most of our customers are in the medical industry. However, we’ve come across specific challenges in achieving robust support for this protocol:

  1. Integration with Telnyx: We’ve found that our PBX does not accept t.38 invites when using Telnyx and other trunk providers that support t.38. This has led to difficulties in ensuring reliable fax transmissions.
  2. Issues with ATA: Similarly, when utilizing an ATA, we noticed that the ATA sends a t.38 invite, which is then declined by the PBX. This again hampers our faxing capabilities.
  3. Helpdesk Responses: On seeking assistance, the feedback from the helpdesk has sometimes been non-specific, missing the particular nuances of our issue. There have been extended delays between our queries and the responses, which impacts our operations.

Given the reliance on paid support, our expectation was for a more streamlined and precise troubleshooting experience.

By addressing this here, our hope is to not only seek a resolution but also to contribute to the community by sharing our experience. We believe that transparency can pave the way for constructive solutions and would appreciate insights from both the community and the VitalPBX team.

If anyone has encountered similar challenges, especially concerning T.38 for SIP faxing, or has any insights into resolving these, your input would be invaluable. We experienced these issues with the lastest version of VitalPBX 3 and 4.

Thank you for taking the time to understand our concerns. We’re optimistic about finding a path forward together.

Warm regards,

Bump. I am still hoping for some of feedback from someone.

I had issues with T.38 so i got rid of it.
If the connection is fine, even multiple pages on faxes are okay with default g711.

What ata is it exactly? With the Grandstream one i can give hints:

  • 3x: Disable Call-Waiting to YES
  • Fax Mode to Pass-through
  • Jitter buffer to fixed and high
  • Disable Line Echo Canceller (LEC): Yes
  • Disable Network Echo Suppressor: Yes
  • Optional: (Re-INVITE After Fax Tone Detected: → Disabled)

Thanks for your reply

Faxing (outbound) without t.38 won’t be considered for our production environment.

Using g.711, when sending over 2-3 pages, the success rate plummeted to about 50% which is unacceptable.

We are able to send faxes with an ATA using 3cx and t.38 (3cx has a pass-through mode). Works well, but we have some connectivity issues caused by our customer’s restricted network security (medical clinics). Cloud based PBX

We were hoping to use OpenVPN to connect directly to the PBX but 3cx does not support OpenVPN and their “SBCs” don’t support t.38 pass-through.

OpenVPN works well with VitalPBX and an ATA but no t.38 faxing.

So in short:
3cx PBX = T.38 “Yes”, OpenVPN “No”
VitalPBX = T.38 “No”, OpenVPN “Yes”

This is your real problem.

For example, in some Canadian provinces where we operate, by law, all dental clinics have to be behind a VPN for security reasons. We had to work with a 3rd party provider to have them add exceptions to the firewall to bypass the VPN and do these changes to every end user’s Sonicwalls. It was very time-consuming on our part and would always break at one point when they did other changes.

I totally understand. Fax can be a hassle with voip this is why we don’t give any warranty on it.
Of course, it’s still interesting if you can get T.38 to work in a positive way.

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