We frequently hear from Power-T.38 customers who have created a SIP trunk and connected their equipment to it, but don’t really have a convenient way to test it. If that’s you, you’ve come to the right place. You could, of course, pull a handful of random fax numbers from a few corporate websites and send a couple of test faxes to those numbers, but that only tests outbound faxing—and it’s also a bit unfair to the receiving party, since they have not invited you to use their number for this purpose. Don’t worry, we’ve got your back!
Fax Test Service
We offer a simple fax test service that receives any fax you care to send to it and then promptly faxes the entire received document back to you. This allows you to test faxing in both directions, and will also show you exactly how your original fax looked when it made it to the other side.
Step 1: Send Test Faxes to 484-260-9009 and 484-260-9109
To test any fax device you’ve connected to a Power-T.38 SIP trunk, send at least one fax to each of these numbers and within a few minutes of each call ending, you should receive an incoming fax from our test facility that will contain an exact copy of what we received from you. If everything looks correct, give yourself a high five and take the rest of the day off because this stuff is not easy! If you have any problems at all, proceed to the next step.
Step 2: Troubleshoot Failures
If your outbound call fails to connect, or does connect but fails to successfully transmit your fax to us on the first call, you may have a serious low-level problem. Double-check your settings, review any diagnostic logs from your fax device, collect and analyze a Wireshark packet capture, and if all else fails, contact our support team. Oh, and don’t worry if you don’t know what Wireshark is—we do!
If your outbound fax transmits normally but you don’t receive a fax in response within five minutes, there’s a few likely culprits:
- Outgoing Caller ID problem: Our test service records the Caller ID from your original incoming call and will send the return fax to that number. If your outgoing Caller ID is incorrect or invalid, we may be trying to send our reply to the wrong number. Review the “Inspect Outgoing Caller ID” section below to make sure you’re setting this correctly.
- Incoming fax problem: If you’re sure we’re getting the right Caller ID but you still see no incoming fax, you may have a low-level incoming call problem. Try calling your device’s fax number from a cell, office, or landline and see if you hear fax tones. Keep troubleshooting until you do get that working, and then repeat your fax test.
If all else fails, remember we have a highly trained team of technical support engineers standing by to help you get this working.
Once Faxes Are Working
A successful fax test is a great start—and you should feel free to do a victory lap around your office (or home office) to celebrate—but when the euphoria has worn off there’s a few more things you should check, just to make sure you’re set up for success over the long term.
The items in the following list share two common characteristics: they’re easy to overlook, and they can also lead to significant reliability and performance issues over the long term. Fortunately, they’re also pretty easy to fix, so let’s get started!
Check Our Knowledge Base
Our interop and support teams have dedicated considerable time to testing and crafting comprehensive configuration guides for many of the more commonly deployed soft-switches, gateways, ATAs, IP-PBXs, and fax servers available in the market. You never know what valuable insights might be hidden within these guides. So, take a look at our knowledge base for your product and if you find a guide, use it to double-check your configuration.
You can follow our guides exactly if you're connecting your fax endpoint directly to a Power-T.38 SIP trunk. Real-world deployments often involve intermediary hops through PBXs, SIP proxies, media gateways or SBCs, but even then, most of the fax communication is passed directly through to us and so we recommend only deviating from our guides when absolutely necessary.
Verify Session Parameters
Most fax machines and fax servers will be able to tell you the speed and resolution used, and whether ECM was present on the transmission. Optimum session parameters will be 14,400 bps, with 2D-MMR image compression and ECM error correction. If you only get 9,600 bps and/or don’t see ECM being used, you should look to make sure this is not being limited by the T.38 endpoint or the fax machine on the other side. Our fax test service is the most reliable way to test your session parameters, since it will always offer the optimum that can be achieved on each call.
Confirm T.38 FoIP is Being Used
Our product offers reliable fax transmissions over SIP trunks by using the T.38 protocol, which changes audio fax tones into data that can be safely transmitted over SIP without any risk of audio signal loss. It is important, therefore, to make sure calls are using T.38.
Many fax devices include diagnostic tools that will tell you if T.38 was successfully negotiated and used on a fax transmission, but the industry standard way to do this would be to collect a packet capture of a successful fax transmission, open it in Wireshark, and pull up the SIP ladder of the relevant call. You should see an initial INVITE (which is the call getting set up), but then a bit later, still in the early stages of the call, you should see a reINVITE to T.38 from party who receive the initial INVITE, followed by a 200 OK from the other side followed by a switch from G.711 RTP (audio) to T.38 UDPTL (non-audio data).
If this all sounds a bit too technical, remember, we’ve still got your back. Simply ask our support team to confirm that they’re seeing your sessions use T.38. Please be sure to provide them with a specific successful fax “call sample” to look at, which is the calling number (CallerID), called number, date and time, and approximate duration of the call.
Send a Really Long Fax
It’s not unusual to have some SIP device or firewall in the call path impose some sort of session timer that closes the connection after a certain interval. For instance, it’s quite common to have SIP session timers be invoked at the 15- or 30-minute mark to check to make sure a call is still going, and to tear it down if it’s unable to renew/refresh that timer for any reason. Sometimes the culprit is aggressive NAT session timers that close the NAT session that was keeping track of your public/private IP pair five minutes after it was created. For these reasons and so many more, we recommend you send at least one fax with a duration of 35 minutes or more. This will help to make sure you’re not going to get a bunch of faxes failing at the 30-minute mark once your trunk is in production.
Test Concurrency
Power-T.38 SIP trunks are elastic and will support as many concurrent fax sessions as you care to transmit, but it’s possible to configure a hard limit on concurrency in the trunk configuration section of our customer self-care portal. The default for activated customer accounts is 20 sessions, and free trials are limited to 8. Whether you have a 2-port analog gateway connected to two fax machines or a 48-channel RightFax server, it’s important to set an appropriate maximum concurrency value on each trunk, and to perform at least one test with all channels in use. An easy way to do this would be to send outbound faxes to your T38Fax fax numbers, effectively faxing to yourself and thus using one outgoing and one incoming session for each concurrent fax transmission.
Inspect Outgoing Caller ID
If the outgoing CallerID matters, you should confirm it matches your expectations. An easy way to do this would be to send a fax either to your cell or to a home landline or office phone that displays CallerID. Don’t be fooled by the TSID (Transmitting Station Identifier) that gets exchanged as part of the fax protocol and often shows up on the top of each page of the fax image, as that’s something programmed into the fax device that may or may not reflect the actual CallerID that is being set at the telco level.
Other Testing Ideas
Whenever we get involved in onboarding a new customer, T38Fax’s standard test procedure involves sending the following test faxes:
ten 1-page faxes
five 10-page faxes
two 50-page faxes
two 100-page faxes
Each of these test faxes should be successful in a single call, and any failures should be investigated.
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