GXW4248 and SIP


So I have been using GXW4248 with Free PBX for several years and have and nothing but trouble with FAX, CC, Modem, etc… Everything else works great but reliability on especially on FAX has been a disaster. I haven’t been able to narrow down the issue to a specific component and I have tried a lot of things, over and over. I have 9 PBX’s and about a dozen GXW’s connected. At this point I would be happy to even get T.38 (14400) working for fax if it was reliable. So I was thinking, I’m not using the Free-PBX for anything other than mapping phone numbers to extensions (i.e. account # on GXW), isn’t there a way to do this via SIP without the PBX? I don’t need the ability for extensions to call extensions, never happens anyway.

Is there any document (or preferably video) that shows how I can setup my SIP from Telnyx directly to a GXW4248 without a PBX? I am ok with mapping phone numbers directly to an account (port) on the GXW and I have no issue with creating 1 SIP channel at Telnyx per GXW.

I’ll still use my PBX for regular phones and extensions but for Fax, alarm, and CC machines I think I need to streamline things?


Using FreePBX for Fax is a nightmare. I would not do it. I would use ATA for regular analog phones but not fax. I use t38fax.com for my faxing and it works great.


actually, I’m trying to get it set for credit card machines and fax machines. If my customer has a fax machine how do I connect it to t.38 service? do they use a POTS/T.38 converter connected to my fax machine? I’m trying to do lots of lines in a mall, I dont want to setup an account for each store, I already have copper cabling 66 blocks and the GXW4248 is installed and working for everything else. I just want the t.38 in the environment to work as advertised. T.38 is T.38, t38fax.com isnt running a specialized version, they just figured out how to set it up. I suspect the issue is between Telnyx and FreePBX which is why I am trying to setup a GXW4248 to connect to Telnyx via SIP directly,.


Here is a video from CrossTalk on setting up the ATA https://youtu.be/P2euqqOOLu0 . They highly recommend not to use FreePBX for faxing. Not every SIP provider fully supports the T38 protocol or at least support it well. I havent had a single issue with them and was having tons routing my faxing through Flowroute and Voip.ms. I use the gw HT series as far as ata’s. Your going to need an ATA for the analog lines.


Thank you for the video link. I love Chris’ videos, I have watch every one he made for FreePBX! He definitely puts out good quality videos showing exactly how to do things in FreePBX and that’s how I got started a few years ago with his video series. I exclusively use Grandstream now for phones and ATA’s. Wish they came out with a newer 4008 but the 4248 have served me well in general. Telnyx claims to be a carrier grade SIP provider (they are pretty big) but they bought or merged like 2 times since I started using them and went through several months of growing pains, the DDOS attacks didn’t help them either.


So no one knows of a document that shows how to setup a GXW4248 directly to a SIP service like Telnyx? Can a GXW4248 talk directly to the SIP server or do I have to have a PBX in the middle? Mostly I don’t understand how to link/assign a phone number to an account (port) on the GXW without a PBX?


I suspect it can be done. The Profile settings would have the general Info for the SIP server (Telnyx). The FXS settings would all use the single Profile for Telnyx, as this tells the GXW where and how to send to Telnyx, but the user ID would be the DID, the AuthID a trunk AuthID and the password would be the password for the trunk. In essence each FXS would be a Telnyx trunk with 1 DID associated to each trunk/SIP connection. The latter part is what makes each line unique.

t.38 FoIP is a pain and I tend to doubt that the elimination of FPBX will solve anything; unless FPBX is underpowered to handle the existing telephone voice load with Fax. The primary issue with t.38 or faxing in general is the networks in-between and has nothing to do with the endpoints. t.38 is a standard and while there may be some design elements in a given manufacturer’s product that may make it more or somewhat less robust than someone else’s product, that difference is small provided the understanding and setup of the devices are the same. Also, 14.4K only increases the odds of failure in most cases. Just remember, that unless the origination and destination are on-net, then the switch from a TDM network to a packet switched network will add more complexity as the data packets wind their way from your ISP to internet carriers that hand off the packets to the ISP at the other end and then to a fax machine. Packets may drop, arrive out of sequence or with such latency that it is not possible for the packets to be reassembled into a meaningful received fax image. t.38 will try and overcome these issues, but it too has limits.


Thanks again for the response. I suspect its actually an issue between the PBX system and the SIP carrier. From my PC (where the GXW is) I can ping the PBX < 21 ms, I can ping telnyx in <20ms. I am in NY and both the PBX and Telnyx hub are in WI (1000 miles from me but LESS than 90 miles from each other) but if I ping Telnyx from the PBX its 42.2 ms??? does not make any sense! Both say its not their issue.

I did a lot of experimentation and the faxes usually fail in 50-70 seconds but occasionally work. I have been able to send 20 pages and 90% of the time I cant send 1.

Just as important I need tobe able to use credit card machines (usually they run at 2400-9600 baud) I assume this uses T.38?


Distance, while an indication of latency is not the only factor. It has to do with the routing and what carriers are used in that same. For instance, you may be using Xfinity as an example, and the route is using Cogent. The PBX maybe using Spectrum and the carrier could be Sprint. I am not suggesting that Cogent or Sprint or Century or any other carrier is inferior to another, but the routing that one takes to get from point a to b in one instance is not the same as another. 42ms is still an acceptable latency. The fact that you were able to send at all lends some credence to the notion that FPBX is likely more stable than the network path between Telnyx and the network path to the destination after leaving Telnyx.

t.38 is a fax protocol, credit card machines and the like use modem protocols such as v.32, v.34, etc. They also take an analog audio sound and convert into digital streams. What each does in order to convert and then transmit/receive is different. You can expect problems with the CC machines as well. Again it comes back down to the differences between a TDM network (time-division multiplexing) method over a single solid connection versus a packet switched network where each packet in a stream leaves at a constant rate and in order, but may not arrive at the destination in the same manner as the packets may take different paths.