VoIP phone services are mostly incompatible with a fax transmission.
Some networks (and fax manufacturers) have tried to adjust to allow for compatibility, however even in the best case scenario a reliable connection is not guaranteed.
This page describes the best way to try to connect fax with a VoIP phone service.
Our first recommendation to faxing with a VoIP line are the following alternatives:
- an online fax service.
- a fax machine (or a computer with fax software) attached to a land-line phone line.
- using a local retail outlet with a fax machine.
Why VoIP and fax are incompatible
Most VoIP phones will scramble a fax signal so that it can’t be understood by the fax machine on the other end of the transmission.
To be able to connect a voice call over the internet, VoIP uses compression that can distort a fax transmission so much that the fax machine on the other side can’t understand it.
We cover more on this topic on our page “Why does my fax machine not work with my VoIP phone line?”
Sending or receiving a fax with a VoIP phone service
Here’s our guide to trying to troubleshoot connecting a fax machine with 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.
We also have VoIP specific guides for: Skype
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.
Some VoIP systems will have a telephone port, others will need an adapter to connect the fax machine to their system.
To make sure that the fax machine will properly connect, connect a traditional landline phone to the VoIP phone and test to see if a call can be made.
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.
We keep a list of test fax services where you can fax and get a faxed reply.
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 – use our list of test fax services if necessary.
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 regular full speed and transmission quality.
If the VoIP system had you make a setting change, turn it back to where it was before trying the fax machine – it could make some of the VoIP system delivery more difficult.