HT814 and faxing over IP


We just moved to voip and purchased the HT814 to handle our analog fax machines. Unfortunately, we are getting transmission and time out errors.

I found this older post about settings, but it’s not an exact match for the 814, and it’s several years old

Are there any updates to this available? What is the current recommendation for configuring the 814 for use with fax machines over voip?


Does your provider expressly state that they support t.38?


Yes, they do support T38.

I just got this from Grandstream support:

Thank you for contacting GS Support.

can you try these settings?
– Tune the Fax BAUD to be less than 14400, 9600 or less recommended (done on the fax machine)
– Set FXS PORT --> Use First Matching Vocoder in 200OK SDP to YES
– Set FXS PORT --> Preferred Vocoder: choice 1: PCMU
– Set FXS PORT --> Fax Mode to T.38
– Set FXS PORT --> Re-INVITE After Fax Tone Detected to “Enabled”
– Set FXS PORT --> Disable Line Echo Canceller (LEC) to YES
– Set FXS PORT --> Jitter Buffer Type to Fixed (if that causes issues you can set it to Adaptive)
– Set FXS PORT --> Disable Call-Waiting: set to YES
– Set FXS PORT --> Disable Call-Waiting Caller ID: set to YES
– Set FXS PORT --> Disable Call-Waiting Tone: set to YES
Also, Check with your carrier if they support T.38, if not change Fax Mode to Passthrough

I have made these changes (had to put Jitter back on Adaptive, though) and so far it’s been working. It’s only been a few hours though.


There are plenty of providers that claim they support T.38, but their t38 implementation is garbage.

I highly recommend attempting to just use passthrough mode, no T.38. Very few carriers fully support T.38 properly - It gets worse with newer fax machines too, since newer fax speeds and commands are available on the market but not available within the T.38 spec.


Try a multi-page fax and several while the office traffic is high. FoIP is hit or miss regardless of t.38 or using pass-thru g711 mode.


There are plenty of providers that claim they support T.38, but their t38 implementation is garbage.

OK. I have set freepbx to T38 Pass-Through with no error correction.

I was wrong about the trunk - there is no t.38 support there.


since newer fax speeds and commands are available on the market but not available within the T.38 spec.

I have already set the fax machine itself to 9600 baud


Now for the fun.

While some providers may not support t.38 and others not very well, t.38 is considered to be superior over a voice codec which is what the pass-thru will use.

As I stated earlier, the ISP and network in-between is the key aspect as it all about the quality and reliability of the connection such that the streams can make it from A to B with very, very little alteration.

In my area, such is hard to find and as a result, I will no longer attempt FoIP after trying almost every ATA known to man over the past 10 years. Some are certainly somewhat better than others and there is some hype with a few, but it all boils down to the delivery. If you have a good connection with decent routing thru the path, then any ATA will work.

There are a couple of providers that seemingly support t.38 well and I have used them for such without much issue, but these were sites using fiber with plenty of bandwidth headroom and in larger cities (KC, Omaha, Houston and Austin).

For my area, I use HTTPS fax or PSTN or even a EFAX type of solution. It is simply not worth the frustration and time.


Well, I guess time will tell. One fax line has ported over so far and it’s our main one. The others we have aren’t used nearly as much.

Since making the changes yesterday we’ve had no errors. If that changes we may need to look at other options. I’m keeping my fingers crossed.


For fun, if you have another ATA we can play with, I’ll give you a free SIP trunk to test on our system. My engineer and I wrote our own T.38 stack because the Asterisk built-in one was garbage (not worked on since 2006 or something), so I’d be curious if it works better on our T.38 platform or not.


We only have the one HT814, so no way to ‘play’ on other systems.

FYI - we just successfully sent an 84 page fax. Well… It took two tries and almost 90 minutes, but it did go through. I told them to not send such huge faxes in the future, to break them into 10 page groups.

Maybe some day that remote site will get Internet access and we can just send emails to them…


Excellent :+1:


Problem with fax is that any error stop it all.
84 pages it is miracle to pass :slight_smile: