Solved T.38 Faxing

[jwmilleril]

I know there are a ton of articles on this forum about T.38 faxing but I'm have a devil of a time getting it to work consistently. We use Telnyx as a SIP provider which supports T.38. I have all of the parameters set correctly on their side per the configuration guide. But they are saying the 3cx server is not sending a T.38 re-invite. I've tried many different configurations including turning off the "Enable G.711 to T.38 Fallback" box (and restarting the IVR service after the change). Do I need to check the box under the SIP trunk options that says "Supports Re-invite"? The how to guide suggests that i should leave that disabled. Incidentally, I've tried it both ways and it doesn't seem to matter. Any suggestions?

 

[cobaltit]

I've not used Telnyx but we'd done T.38 on a number of providers without issue with out of the box settings. But if this is for anything important I'd advise using 3CX T.38 faxing (or any other PBX faxing) for anything other than the occasional low-volume use

 

[chance-wmt]

We use T38fax.com for faxing, due to the fact that 3CX faxing via ATAs does not work reliably via VPNs and not at all via SBC. Faxing is one of those things that we try to get the customer to move away from first anyways, setting up faxing is not always easy. I wish 3CX would focus on faxing, but honestly it's a dying technology. On that note, I highly recommend a Cisco SPA112 for a fax ATA adapter.

 

[YiannisH_3CX]

Hi @jwmilleril

For the 3CX fax server to send a T38 Reinvite you need to keep the "Enable G.711 to T.38 Fallback" option off and point the call to a Fax extension of the PBX (or to the email of a regular extension).

Do I need to check the box under the SIP trunk options that says "Supports Re-invite"? The how to guide suggests that i should leave that disabled.

The option should be disabled. The PBX will send the re-invite anyway.

 

[jwmilleril]

Forgive my ignorance on this, but when I disable the G.711 to T.38 Fallback option (uncheck the box) and restart the IVR service, the system no longer sends a tone. Maybe this is by design? So, if I then try to send a fax over a POTS line, the fax machine never hears the tone. I'm confused how my T.38 Gateway provider (Telnyx) would make the fax machines connect if no tone is sent?

 

[chance-wmt]

Also, on your sip provider. Insure your sip provider has RTP auto adjust disabled on your fax number and that they have T.38 enabled with re-invite initiates at customer end.

 

[chance-wmt]

Also, on your sip provider. Insure your sip provider has RTP auto adjust disabled on your fax number and that they have T.38 enabled with re-invite initiates at customer end.

This is something you can do on Telnyx.com

 

[jwmilleril]

Thanks. I had all of those set correctly.

 

[chance-wmt]

Also, I noticed that if the sender is using ECM it fails on 3CX.

 

[YiannisH_3CX]

Forgive my ignorance on this, but when I disable the G.711 to T.38 Fallback option (uncheck the box) and restart the IVR service, the system no longer sends a tone. Maybe this is by design? So, if I then try to send a fax over a POTS line, the fax machine never hears the tone. I'm confused how my T.38 Gateway provider (Telnyx) would make the fax machines connect if no tone is sent?

In this scenario the PBX will switch to T38 immediately and will send a T38 re-invite as soon as the call reaches the Fax server so you will not hear any tone. Since Telnyx supports T38 bypass, it will pass the re-invite to the sender. If the sender does not support T38 the Fax transmission will probably fail. In that case you will need to enable the G.711 to T.38 Fallback option so the Fax can be transmitted through audio but that significantly reduces the chances of success. But since Telnyx wants to see a re-invite the option should remain off.

 

[jwmilleril]

I moved all of my efax lines to a SIP trunk I created on T38FAX.com and then connected the ATA devices directly to T38FAX.com (not through the 3CX server). All faxing works perfectly now.

 

[chance-wmt]

I moved all of my efax lines to a SIP trunk I created on T38FAX.com and then connected the ATA devices directly to T38FAX.com (not through the 3CX server). All faxing works perfectly now.

Glad you seen my recommendation. Their support is unbeatable, someone pickups the phone within minutes and is always able to help and test. I wish some of these SIP providers did the same.

Regarding 3CX faxing, at this time. They do not still support functionality over VPN or SBC. This is why I believe it is fair of me to recommend a different solution than 3CX. Other than that, 3CX is as good as it gets.