Why fax does not work on NBN
Traditional fax machines rely on an analogue phone line. NBN replaces the copper phone network with a digital connection, so the analogue signal a fax machine expects is no longer available from the wall socket. If you plug a fax machine into the phone port on an NBN modem, it may dial but will typically fail with a communication error because the modem converts voice to VoIP packets, which are not reliable enough for fax timing.
The problem affects all NBN connection types - FTTP, FTTN, FTTC and HFC - because voice is delivered over VoIP in every case. Some ISP-supplied modems handle fax better than others, but most do not support it reliably. For example, the NetCom NF4V failed in our testing while a dedicated VoIP adapter worked.
How to make fax work on NBN
The most reliable solution is a dedicated VoIP phone adapter connected to your NBN modem via Ethernet. The adapter converts VoIP back to an analogue signal that your fax machine can use, with codecs and timing optimised for fax transmission.
Recommended adapters
- Grandstream HT801 or HT802 - currently available single and dual-port VoIP adapters with T.38 fax support. The HT802 provides two phone ports if you also need a handset.
- Cisco SPA112 - the adapter we originally tested and confirmed working. Now discontinued by Cisco, but still found second-hand.
Setup steps
1. Connect the VoIP adapter to your NBN modem with an Ethernet cable.
2. Disable the built-in VoIP account on the NBN modem so it does not conflict with the adapter.
3. Enter your ISP VoIP credentials (SIP server, username, password) into the adapter.
4. Connect your fax machine to the adapter phone port.
5. Test by sending a fax to 1300 368 999 - this Australian test number accepts your fax and returns it with transmission details and error rate.
VoIP adapter settings that help
Use the G.711a codec (the standard for Australian voice). Enable T.38 fax relay if your ISP supports it - T.38 is a fax-over-IP protocol designed to handle the timing issues that break fax on regular VoIP. If fax still fails with T.38 enabled, try disabling it and lowering the baud rate to 9600. Disable ECM (Error Correction Mode) as a last resort, since some ISP VoIP implementations do not handle ECM retransmissions well.
Online fax services as an alternative
If you do not want to deal with hardware, online fax services send and receive faxes via email or a web portal. You get a fax number and incoming faxes arrive as PDF attachments. Options include eFax, fax.plus and Fax2Email. Monthly costs vary but are often comparable to the cost of a phone line dedicated to fax.
Back-to-base alarm on NBN
Traditional back-to-base alarms use the analogue phone line to dial the monitoring centre when triggered. NBN removes that line, so the alarm either cannot dial out or becomes unreliable.
Solutions for alarms on NBN
- 4G/5G cellular communicator - the most common solution today. A small cellular module is added to the alarm panel and sends alerts over the mobile network, independent of your internet connection. Most security companies now offer this as standard.
- IP alarm communicator - connects the alarm panel to your NBN modem via Ethernet and sends alerts over the internet. Less reliable than cellular if the internet drops out.
- Connect alarm after the NBN modem - plug the alarm into the phone port on the NBN modem. This works in some cases but is not guaranteed, especially if the modem loses power during a break-in.
Contact your security monitoring company before NBN is connected. They can install the appropriate communicator so there is no gap in coverage.
Multiple phones and devices on NBN
NBN modems typically provide only one or two phone ports. If you have extension phones, a fax machine and an alarm all connected in parallel, some devices may not work reliably. Keep the NBN line from the street connected only to the NBN modem input. Then connect your devices to the modem phone port or, better, to a VoIP adapter. The success depends on the number and type of devices sharing the line.