FAX to some numbers are not working

[dustycr]

Hello all,

since two days our fax is not working but only when sending to specific numbers.
Following the setup

Printer / FAX Machine -> Grandstream HT701 -> 3CX

Our provider is easybell which is not accepting t38 protocol, so it's disabled in the the grandstream options and set to passthrough mode.

When a fax being sent to one of the not working numbers i can see in the activity log that 3cx sends the following t38 options

a=T38FaxVersion:0

a=T38MaxBitRate:14400

a=T38FaxRateManagement:transferredTCF

a=T38FaxUdpEC:t38UDPRedundancy

Aftet that im getting the following message:

Sending: OnSendReq Send Req BYE from 0.0.0.0:0 tid=38342050ba42355e [email protected]:

BYE sip:[email protected]:5060 SIP/2.0

Via: SIP/2.0/ ;branch=z9hG4bK-524287-1---38342050ba42355e;rport

Max-Forwards: 70

Contact: <sip:xxxxxx>

To: "FAX"<sip:[email protected]>;tag=1936181727

From: <sip:[email protected]>;tag=39216126

Call-ID: [email protected]

CSeq: 3 BYE

Reason: SIP;text=Application Rejected Sdp(usually no common codec)

Content-Length: 0



 Offer is rejected on Leg L:1838.1[Fax:80];
OnOfferRejected Recv 488/INVITE from 10.10.10.25:5060 tid=719cd542325bdd09 [email protected]:

SIP/2.0 488 Not Acceptable Here

Via: SIP/2.0/UDP 10.10.10.1:5060;branch=z9hG4bK-524287-1---719cd542325bdd09;rport=5060

To: "FAX" <sip:[email protected]>;tag=1936181727

From: <sip:[email protected]>;tag=39216126

Call-ID: [email protected]

CSeq: 2 INVITE

Allow: INVITE, ACK, OPTIONS, CANCEL, BYE, SUBSCRIBE, NOTIFY, INFO, REFER, UPDATE

Supported: replaces, path, timer, eventlist

User-Agent: Grandstream HT701 1.0.8.2

Warning: 304 GS "Media type not available"

Content-Length: 0

In working fax calls the t38 options are not sent.

Has this problem something to do with the connection between voip and normal phone lines ?

 

[sip.bg]

No, I think you must use a provider supporting T.38.
In other case, i.e. G.711 passthrough the results could be less than 50% success rate, depends on many factors like latency, jitter, packet loss.
In settings | fax server try to enable G.711 fallback to see if any improvement happens

 

[dustycr]

For incoming faxes we dont have any problems its just outgoing to some numbers.
Or does the fax server option G.711 fallback affect outgoing faxes too ?

 

[sip.bg]

For incoming faxes we dont have any problems its just outgoing to some numbers.
Or does the fax server option G.711 fallback affect outgoing faxes too ?

Try it, I haven't found any documentation. The status is experimental.
I think it affects only ATA used and not the SIP trunk.
The PBX is transparent for T.38 operation - both SIP trunk and ATA must support T.38.
If fallback is enabled, probably the adapter will be able to renegotiate T.38 to G.711 and vice versa.
In FreePBX for example the most stable combination is T.38 capable trunk and G.711 ATA.

Make a wireshark captures to investigate the reasons for failures and examine also the fax logs.

 

[lneblett]

The server aspect where you set for fallback to g711 is only a receive function. Did you set the ATA as a regular extension or a FAX extension?

Maybe this might give some insight.

https://www.3cx.com/voip-gateways/

 

[us1]

Try bypassing the 3cx system and register the ATA directly with the SIP provider. This should help reduce a hop in the chain and further determine the source of the problem.

 

[dustycr]

The server aspect where you set for fallback to g711 is only a receive function. Did you set the ATA as a regular extension or a FAX extension?

It was registered as a Fax Extension.

Try bypassing the 3cx system and register the ATA directly with the SIP provider. This should help reduce a hop in the chain and further determine the source of the problem.

I will definetly try that later and check if it helps. Thanks !