Intermittent Issue receiving Faxes

[Neil @ CommsProvider]

Good morning,

We have a customer using 3CX (v16 update 7 on Windows) and they are having intermittent issues receiving faxes. They are a solicitors so receive mortgage information via fax. One bank can send through fine, another cannot.
I have done testing by sending a fax to their system, and ours fail. I have wiresharked it and the only think i notice that it does an initial rtp handshake on one port, but then all replies from the 3CX are on the next port up. Is this expected behavior?
I have had the SIP provider look at it with me and they can see all the messages they expect so don't believe that it is an issue at their end. I am absolutely stumped as to what is causing the issue, and am struggling to capture a wireshark of a working fax (to compare) as they don't know when they are coming in to let me know.

For more info, the faxes come in on separate DDI, and we are using the 8888 fax extension to answer the call. I have put a attachment of the 'failed' fax below (left is their SIP SBC, Middle is the 3CX, right is the SIP RTP SBC)

Any suggestions welcome

Cheers

 

[NickD_3CX]

Hi there!

About the changing of the port, yes, this is normal. Every fax call starts off as an audio call and once the 3CX Fax Server answers it, it will send a re-INVITE message asking to continue the "conversation" in T38 on a new port.
This is absolutely allowed by RFC.

The most common problem we see with faxing is that, after 3CX sends the re-INVITE and switches to the new port, the Provider continues sending packets to the old/initial port.

From your screenshot, you can see that 3CX is sending some packets but not getting any response, so it could be this case, but you would need to check.

Which provider are you using? Is it one from the Support Providers page?

 

[Neil @ CommsProvider]

Hi,

Yes i believe that is what it is doing. We use Gamma for SIP trunking so is a supported provider, could they have changed recently as the issue has only started happening in the last month or so.
Is there a chance the issue is on the sender provider, as i am still unsure as to why some faxes can get through and others cannot.

Many thanks

 

[YiannisH_3CX]

Hi Neil,

We investigated a couple of issues in the past with Gamma and intermittent fax issues and although we were never able to replicate the behavior, it looked like fax coming from one IP block worked while fax coming from a different IP block failed.
So i would recommend capturing a few working and a few failed attempts and comparing the IP source. You could run a tshark command that allows wireshark to capture data constantly and save them in smaller parts so it does not put stress on your PBX.

 

[Neil @ CommsProvider]

Hi,

I will give the tshark command a go. Once i get a good example i'll take it to Gamma and see what they can do for us

Cheers

 

[SKDamon]

Is "Enable G.711 to T.38 Fallback" active under Settings -> Fax Server?
I've had trouble in the past with receiving faxes, all fax calls stopped after 10-12 seconds.
For me, 3CX Support recommended to set FAX_PCMU_T38_FALLBACK_TIMEOUT to a higher value.
If you see the same behavior (all failed faxes being 10-12sec calls), you might give that a show.