Can't use t.38 - Get Status "488 Not Acceptable Here" everytime. (V4)


I am trying to send faxes using t.38 on trunks that supports it and VitalPBX will reject the t.38 invite.

I am able to send small documents (using regular g.711) but when it gets more and more unstable after 2 pages obviously.

Trunks providers I tested with are Telnyx and’ special fax trunks

These are my current settings:

These are the Endpoint settings

What am I doing wrong? Thank you for your assistance

Here’s the right trunk configuration for sending faxes using Telnyx with the registration method.

Moreover, you need to configure the NAT settings in the PJSIP Settings module.

By the way, you can use “” if you are located in the USA or depending on your Telnyx account configurations.

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