Can I use a non-fax modem with T38Fax?

  • Updated

Aside from fax machines, there are many other kinds of modem devices on the telephone network.

  • Fire alarms
  • Credit card terminals
  • ATMs
  • Dial-up modems
  • Many more...

Since the behavior of modems is extremely variable - sometimes proprietary - and the problems that modems face have been largely resolved by cellular data connections in the latest devices, no open standard has been developed for transporting non-fax modems over the internet more reliably than with a standard VoIP call. That said, there is still some demand for using modems over VoIP. There are still many legacy devices using modems connected to phone lines, as well as devices that must use a phone line due to poor cell reception.

You're more than welcome to use a non-fax modem on our service. For modem users, we do still offer a well-connected VoIP service and experience correcting call quality issues that specifically affect modems as opposed to voice calls. There are a few limitations, though. The T.38 protocol only supports fax modems, so these other types of modems do not receive the same protection from adverse network conditions as a fax machine. Also, due to the wide diversity and often proprietary behavior of modems in use today, we won't be able to debug the modem or the protocols it uses at the same level as a fax call, but your modem vendor may be able to help with that.

When configuring your ATA: the ATA jitter buffer adjustments in our installation guides also help most modems, and we do support all three methods for transporting DTMF signals over VoIP: in-band, RFC2833, and SIP INFO. Some modems will prefer one over the other, but we recommend first trying with the DTMF method set to in-band. Here is a short list of where to find the DTMF method setting in a few popular ATA brands, though most ATAs out there will have a similar setting:

  • Grandstream: Profile X -> Preferred DTMF method (in order)
    Options: in-audio [in-band], RFC2833 or via SIP INFO. [default order]
  • Cisco: Voice -> Line X -> Audio Configuration Section -> DTMF Tx Method
    Options: InBand, AVT [RFC2833] , INFO, or Auto [AVT or InBand, default].
  • Obihai: ITSP Profile X -> DTMFMethod
    Options: Inband, RFC2833, SIPInfo, Auto [Inband or RFC2833, default], SIPInfo+RFC2833 [both simultaneously]

Additionally, always make sure to disable T.38 support on ports any ports servicing a non-fax modem, as T.38 can sometimes interfere with proper modem transmission if your modem is too similar to a fax machine. ATM modems especially are known to have this issue.

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