V16 Calls Dialed Answer With Fax Sound

[redbudindustries]

Linux OS
V16.0.1.273
VMware host
Net2Phone VoIP provider

When we make calls to a particular customer the call rings once or twice and then is gives a fax dial sound. I have tested switching to or backup VoIP provider Twilio for these numbers and they work fine. I talked to Net2Phone support and they were able to register our SIP Trunk and call the number and get through with no issues.
Since the number is a canadian number and is long distance I have tried adjusting the 1 and +1 prepend options but no changes.
I did a packet Capture and sent it to Net2Phone and they sent me the attached image. It show that the 2 invite is trying to negotiate t38 fax.
I have checked all the fax settings in 3CX that I know of and they are either set to default or are unconfigured since we don't use the fax features.

Anyone have any ideas on settings to check or things to try?
Thank you in advance.

 

[JohnS_3CX]

Hi @redbudindustries

You are probably sending the number to the fax server from the Trunk Settings page or something along those lines. Notice the destination here was set as "Send FAX to"

 

[leejor]

What does the 3CX Activity log show for one of these calls? Does it show the call being sent to your provider? If so, is the number sent, the one expected? Does the destination you are calling have a fax machine/service on another number that is similar, or perhaps (unknowingly) have an option to call-forward-busy to their fax line?

 

[craigreilly]

Hi @redbudindustries

You are probably sending the number to the fax server from the Trunk Settings page or something along those lines. Notice the destination here was set as "Send FAX to"

Wrong call direction I think ... They are having problem when they call out.

 

[redbudindustries]

Wrong call direction I think ... They are having problem when they call out.

Hi @redbudindustries

You are probably sending the number to the fax server from the Trunk Settings page or something along those lines. Notice the destination here was set as "Send FAX to"

Craig beat me to it but. but here is our trunk.

We use this trunk for incoming and outgoing calls. It has been configured and worked this way for over a year. As of right now this is the only instance of this issue.

 

[redbudindustries]

What does the 3CX Activity log show for one of these calls? Does it show the call being sent to your provider? If so, is the number sent, the one expected? Does the destination you are calling have a fax machine/service on another number that is similar, or perhaps (unknowingly) have an option to call-forward-busy to their fax line?

As far as I can tell the system is saying that the call is connecting. I have attached a sanitized version of the call log. Have a look and let me know.

Thank you.

 

[leejor]

I'm assuming that the dialled number (that you have changed) remains the same. from the initial Invite to that sent to the provider.
Have you spoken to the called party about any fax machine, or perhaps an alarm system/modem (they can sound the same), that they may have on that line? Does that same thing happen if caller ID is changed at your end, sending a different CID?

 

[redbudindustries]

I'm assuming that the dialled number (that you have changed) remains the same. from the initial Invite to that sent to the provider.
Have you spoken to the called party about any fax machine, or perhaps an alarm system/modem (they can sound the same), that they may have on that line? Does that same thing happen if caller ID is changed at your end, sending a different CID?

Correct, the number dialed stays the same except the outbound rule adds a 1 for long distance code.
I tried to have the caller ID be set by the management system and by the extension but there was no change.

 

[leejor]

I suppose that you could create a Fax extension, have the machine call then seeing if there is a fax answer-back. But, it may not be a fax machine at all. It could be a modem (credit card processor?), or alarm system access. Have you contacted the other end to see if they have reports of other callers having had the same issue? Unless they are screening your particular caller ID, I can't believe that no one else has encountered the data tone.

 

[redbudindustries]

I suspect the fax because of the logs and because the sound that plays on the call is a fax dial. I talk to the Secretary there and she said that she was unaware of anyone else having this issue. I will see if I can get through to their IT dept to get details.

 

[JohnS_3CX]

Sorry did you mean that you get a fax tone when you dial out to that destination (outgoing)?

You can upload the capture you sent your provider somewhere and send me a private message with the link, I would like to see the SIP flow

 

[redbudindustries]

Sorry did you mean that you get a fax tone when you dial out to that destination (outgoing)?

You can upload the capture you sent your provider somewhere and send me a private message with the link, I would like to see the SIP flow

That is correct. I will send you the capture.

 

[JohnS_3CX]

Hi @redbudindustries

As you see here from your capture, all is well up until the call is established.

At that point (packet 9436 in your capture), the provider decides to re-invite you to T.38 which of course gets rejected by the PBX because there is no FAX involved. At this point, your capture does not contain the part of the call between the provider and the other side, so only the provider can see that part and tell you why this happened.

- They say it did not happen when they tested the trunk, but as you can understand from the above evidence, that test tells us absolutely nothing about why this happens to you

- So kindly ask them to investigate further from their own captures while you make a test call from your PBX to replicate the exact conditions