Does T38 Fax support require Internet Access

[ben.meyer]

I have setup a fax server on extension 88 and a CISCO SPA112 on extension 87. I can send a fax from x 87 to x88. When I send a fax from outside the organization to x87 or x88 the line answers but fails trying to negociate. Our ISP has provided a breakout with 2 ports: one is SIP which goes directly to the physical server hosting the 3CX VM. The other port has internet which goes into a Fortigate firewall. 3cx can access the internet through the firewall and our mobile phones run the 3CX application perfectly from outside. My ISP confirms they support 3CX and my firewall engineer says that there is no SIP traffic through the firewall. So no one can help me!

I have run wireshark for both the successful and the failed sessions. The SPA112 is on 192.168.2.42 and 3CX is on 192.168.2.5. I don't understand how the external call skips the status 180 ringing.

I am wondering how T38 works. Does it need some sort of access over TCPIP/UDP that is possibly not configured? How does it handle NAT?

I really appreciate your help.

 

[cobaltit]

So you haven't confirmed if your provider is 3CX approved (sounds like it isn't) nor have to stated whether they actually support T38. You also haven't stated if the firewall checker passes nor what version of 3CX you are running. But no T38 doesn't need anything special.

 

[eddv123]

After checking the above points from @cobaltit care to share what the wireshark captures are saying, I have traced FAX calls before and they are pretty easy to follow, at least to the point where you can pin point where the issue lies.

 

[ben.meyer]

Thank you to everyone who responded. Your help is appreciated. It is great to get direction from folks who have done this before.
It turns out that the teleco did not properly configure their T38 on our account. Sending them the wireshark dumps for successful internal faxes and failed external faxes helped them to figure that out.
Suddenly I am now able to receive external faxes.

 

[eddv123]

Wireshark is always the way with FAX, the other option would of been to try another trunking provider.

It is especially useful if your provider supports the ability for the customer to trace via their platform. My experience of this has been when a FAX leaves 3CX outbound as T.38 and miraculously via the provider network appears as a G.711 call at the other end, sounds like something similar you have hear.

Glad to hear it is now solved however.

 

[cobaltit]

@ben.meyer

Thanks for the update.That's one of the main reasons we became a VoIP provider was to avoid situations like this. I control both sides so I'm not spending time troubleshooting things unnecessarily.