Getting a VoIP phone to work with a fax machine is the largest fax headache we know of.

VoIP phone systems and fax are mostly incompatible. Even in the best case scenario where a test fax is successful, a reliable connection is not guaranteed.

Some VoIP networks claim that fax works and have instructions on how to allow for compatibility, others say it’s not possible.

Recommended alternatives

  • An online fax service like RedConnect eFax Service.
  • A fax machine (or a computer with fax software) attached to a land-line phone line.
  • Printing the fax and going to a local retail outlet with a fax machine.

VoIP and fax are incompatible

VoIP phones will scramble a fax signal as they use compression so that they can work with the Internet.

When the call signal is compressed, the receiving fax machine can not understand the transmission.

How to (try to) send or receive a fax with a VoIP phone service

Here’s some ideas for trying to troubleshoot connecting a fax machine to a VoIP phone service.

We don’t recommend trying this, as even when a system is tested and appears to be working, problems can still happen. Large documents, graphically intense images, and transmissions to other fax machines on a VoIP phone line may cause problems down the line.

What you’ll need

  • A VoIP phone line with a dedicated phone number.
  • A way of connecting the fax machine to the VoIP phone line.
  • A fax machine (or a computer with a fax modem and fax software).

Step 1 – check for any VoIP and fax specific instructions

VoIP providers and fax machine manufacturers may have specific instructions for working with each other’s systems.

In some cases, the VoIP provider may have a preset or option that can be adjusted. Another option is a code to pre-dial before the fax number that sets the VoIP line to expect a data transmission.

Fax manufacturers may recommend turning the baud rate down or turning error correction mode off.

Step 2 – connect the system

Connect the fax machine (or computer with fax software) to the VoIP phone system (ATA).

Some VoIP systems will have a telephone port; others like cloud services will need an adapter (ATA) to connect the fax machine to their system.

To make sure that the fax machine will properly connect, plug a traditional landline phone to the VoIP phone and test to see if it will make a call.

Step 3 – test sending and receiving faxes

The quickest way to test a fax machine is to fax to someone you know and have them fax back to you.

Always check the VoIP provider’s instructions to see if there are special instructions for “fax” or “data” transmissions – some that try to make accommodations for fax have special dial codes that will adjust how the VoIP service handles the call.

You can send a fax to HP’s free test fax number 1-888-473-2963 and it will send a confirmation back if the fax went through

Step 4 – adjust fax machine settings

Most of the time, a fax connected to the VoIP service won’t work out of the box.

There are two ways that fax machines can be adjusted to help with a VoIP phone call.

Reducing the baud rate – setting the baud rate to the lowest possible number (or the transmission rate to the lowest possible speed) will make the transmission easier to understand by the other fax machine. It will also cause the transmission to take longer.

Turn off error correction mode (ECM) – this will make the fax not as clear, but can also make it more likely it will fax through the VoIP phone system.

Step 5 – test the fax again

Repeat step 2 – You can send a fax to HP’s free test fax number 1-888-473-2963 and it will send a confirmation back if the fax went through

Step 6 – return settings (if it didn’t work)

If fax over VoIP isn’t going to work, return the fax settings to where they were – this will mean that if the fax is used later on it will do so at the regular full speed and transmission quality.