3CX and faxing using an ATA - Never ending issues.

I want to start off by how much I don't like fax and why in 2025 we're still using it. No it is not more secure as nothing is encrypted.

Here is our setup.
- SIP Trunk Provider - We are the provider. We use Asterisk with along with a web GUI for tenant and DID management. We fully support t.38 fax standard.
- 3CX - cloud setup, Grandstream HT81X ATA's and connecting via SBC
- ATA - Configured to fax in t.38

Outgoing faxes just continue to fail. What I can confirm in the PCAP captures is 3CX just never sends the request to convert to t.38 nor does it ever respond to the request from our SIP Trunk side. This then causes the faxes to fail to connect and then send.

If I configure the same ATA to connect directly to our Asterisk server faxing works just fine (yes I understand this is a solution). My issue is, why does one work reliably but 3CX appears to be a crap shoot?

For inbound it's the same thing. If we use fax to email I must enable the fallback to G.711 or faxes just don't work.

Has anyone else had luck getting faxes to work consistently?