Fax server is configured but extension 888 answers with silence and hangs up

[Shawn]

I've configured the fax server to receive inbound faxes on extension 888 but when 888 is dialed either internally or through an inbound rule that is configured to take a DID and "send fax to 888", it seems that 888 answers in dead silence and hangs up immediately.

Call history shows the call with a talking time of 00:00:00.

If I understand correctly, 3CX had a built in fax server that is able to receive faxes on the default extension 888 and forward them to an email address as a pdf file. This seems pretty straight forward.

And from what I've learned, outbound faxing is better handled through an ATA device. Am I missing anything here?

We are on 15.5.3849.1

Thanks in advance for any help.

Shawn

 

[Shawn]

Well, I guess perhaps I'm not understanding T.38 faxing. From what I understand now, T.38 faxing is a protocol for fax over IP where my SIP trunk provider needs to be able to handle T.38 faxing so that 3CX can establish a session between the PBX and the SIP trunk and receive the fax after the session is negotiated. I think what I said here makes sense.

Anyway, what I was looking for is 3CX being able to receive a fax from an analog fax machine, so I'm don't think T.38 would play a role in this scenario. I checked the G711 to T.38 fall back (beta) in the fax server configuration and am now able to send a fax from an analog fax machine on a POTS line to the dedicated DID for extension 888. When I call extension 888 from inside, I get the familiar fax tones. I guess what's happening here is the analog fax noises are traversing over a VOIP call and 3CX is decoding the analog noise and converting the received fax to a PDF and forwarding it to the indicated mailbox.

Can an expert comment on my notes here and let me know if I'm understanding this correctly or if I'm way out in left field.

Thanks

Shawn

 

[cobaltit]

You are mostly correct. T.38 or g.711 has nothing to do with what is faxing you. It has to do with what your provider offers. Your provider is already converting the call to IP so the fact that it is coming from an analog fax machine has nothing to do with it. In this case it seems like your provider doesn't offer T.38 (or its not enabled or there was an issue with negotiating it between 3CX and your provider) so it fell back to G.711 which is just fax tones screeching over the audio channel. When you had T.38 only and called it internally it just dropped because there there was no T.38 signaling.

 

[StefanW]

this is normal, if you expect the "beep" sounds to be played, then dont worry, in t38 this does not exist, both parties check if they can speak t38 (and mobile phones or IPphones dont) and upon failed negation they drop the call.

3CX has no outbound fax build in, so an ATA from the supported list is needed and connected to the fax machine you already have.

 

[Shawn]

Thank you everyone for your replies. I understand completely now. I spoke to my SIP provider today and indeed, found out that they do not support T.38. So G.711 seems to be my only option and seems to work fine.

One other question to clarify. So, the SIP providers that support T.38 recognize an incoming fax call and switch to T.38 and 3CX sees the incoming T.38 session initiation and handles the fax receipt from there? I read something about T.38 converting a fax to an image (TIF) and sending it via T.38. Is this correct? So a SIP provider that supports T.38 recognizes a fax call, receives the fax converting it to a TIF, initiates a T.38 session with the 3CX fax server and sends the TIF over T.38 protocol and then the 3CX fax server converts what it received to PDF and emails it accordingly?

Thanks again for your help and knowledge.

Shawn

 

[leejor]

Can't confirm, but I would question...if a provider has already "intercepted" a fax and converted to a TIF (or any other format), why would they not simply email it direct to you? Why resend it on again as a fax?

 

[Shawn]

here's a link where I saw the blip about images
https://www.3cx.com/pbx/t38/

and this is a much more detailed explanation of T.38
https://www.3cx.com/blog/docs/foip-fax-over-ip-t38/

Thanks

Shawn

 

[sip.bg]

Fax is working reliably only if T.38 is used as protocol.

In fact 3CX will work only with T.38 providers, otherwise results are disastrous.
This means you either use built-in incoming fax server or ATA supporting T.38, in this case 3CX is transparent for T.38.
In newest v15.5 sp1 there is experimental fallback to G.711a/µ which means you can use a G.711 compatible ATA (or T.30), still the trunk needs to be Т.38.

Regarding fax tones, you can hear them only on G.711a/µ, i.e. using voice modulation, not on T.38. You may think of T.38 as of exchanging e-mails, you can't "hear" your e-mail attachments.

 

[Shawn]

Thank you for the explanation. I am using 15.5 sp1 and the beta fallback to G.711 is available and checked in my case. My SIP provider is somewhat different than most where they have installed an Adtran router onsite that handles SIP only 3CX sites on the same subnet as the Adtran). In speaking with them, they do not support T.38 however do support G.711. With the G.711 fall back option checked on 3CX fax server setup, I am able to receive faxes (I tested 2 pages on two separate faxes and both worked as expected). The test faxes where initiated from an old multi function printer sitting on a POTS line. I could hear the fax screeching from the fax machine speaker and if I dial extension 888 I am also greeted with the familiar fax initiation tones. I guess G.711 is analog noise over digital medium. I'm not sure what happens on my providers side when an incoming analog fax is detected and what G.711 does when a fax is detected.

Shawn

 

[bbaker73]

I am switching sip providers and was looking at the G.711 to T.38 fallback option too thinking the new provider is not supporting T.38. However, I am getting faxes without that checked/enabled. Does that mean sip provider must be using T.38?

 

[cobaltit]

Yes