Fax Over SIP Trunks

[Rich14]

Hi,

MCD CXI11 4.2 Software.

Do I require T38 licences so send Fax over SIP Trunks(To PSTN).?
Are there any settings on the SIP profile to change.?

The 2 fax machines worked without problem on ISDN30. When the system was moved over to SIP approx 50% failing or working on second attempt.
Fax machines are on analog ports.

 

[IrwinMFletcher]

Do they require it? No. Should you have them? Yes.

As you've found out, faxes MAY work over a SIP trunk, but there is no guarantee.

Note, you'll also need the DSP II octal card as well as the licenses.

 

[Billz66]

we have had some success with using a cisco spa122 as a generic sip extension for a fax

the SPa 122 supports T38 and if set correctly should allow the best chance for faxing on sip.

if fax is mission critical though nothing beats a CO line.
SIp faxing is normally only best effort supported by most carriers

If I never did anything I'd never done before , I'd never do anything.....

 

[mitelmania]

If you are faxing to other PSTN numbers over a SIP service provider I dont think T38 will make any difference, T38 would only be of any use if you were faxing to another PBX that also supported T38. There are a couple of COS options to enable for your fax machine, CAMP ON TONE/FAX MACHINE & BUSY OVERIDE SECURITY, failing that I would try a 128 channel echo cancellation module is bay 3. As its a CX varient controller it will need a shell command to enable it and the command does not survive a reboot.

 

[IrwinMFletcher]

@mitelmania - I'm not sure I 100% agree, or maybe I just don't understand your statement. If the SIP SP doesn't support T.38, then I would agree, there is no point. The call will just default back down to G711. If the SIP SP supports T.38 then it would help regardless of who the destination endpoint is. The SIP SP is the one that will handle the T.38, not another PBX.

And yes, you can try to change the ECs in the system... By default the CX uses the DSP (software based) ECs, these have a different behaviour than those on the dedicated hardware cards (64/128 channel EC card, or the T1/E1 Combo card). In some instances the hardware based ECs don't distort the fax signal as much as the DSP based version, so forcing the system to use the hardware ones may help. There is a software command to use on the shell for this, I thought it was a Maintenance Command too...not sure.