Why does my fax machine not work on my VoIP phone line?

Last Update: August 9th, 2021

The short answer is that fax machines don’t and won’t work over VoIP lines.

Some people have managed to make it work at times. However, there’s nothing we’ve seen that we can consistently recommend for the “general” or “average” user, or even the more daring mid-range to tech savvy users.

The longer answer

The longer story is that the two are incompatible.

VoIP takes the sound of someone’s voice and digitally compresses it so that it can be transmitted over the internet. It turns the sound wave of your voice into computer language (digital 1s and 0s) and then on the other end turns that computer language back into a sound wave.

As fax takes an image of a document, and turns that image into computer language (digital 1s and 0s), that it then turns that computer language into a sound wave that can be transmitted over an audio connection like a phone line. On the other end, the other fax machine then translates that sound wave back into computer language and back into an image.

The challenge that exists is that the way that a VoIP signal converts audio into computer language distorts the signal from the fax machine so that it can’t be understood on the other end – there are too many steps in the process that it just doesn’t work.

When the signal is uncompressed on the other end, the fax machine on the receiving end can’t understand it, and the transmission doesn’t go through.

Fixing the problem

We haven’t been able to find a consistent way of fixing this – in general, a faster internet connection or more technology won’t help – the challenge is how the signal is compressed more than it is if there’s enough “speed” in the system.

Some VoIP providers may have FAQ

For very technical users, the closest we can suggest to a solution is the following:

1) reduce the transmission rate (baud rate) of the fax machine to the lowest setting possible. Usually this will make the fax transmit much rate slower as well as with much less clarity. However, it also makes the signal less susceptible to distortion.

2) enable “QoS” (Quality of Service) through your router for the VoIP service. Most VoIP providers will have some information on how to set this up. This will make it so that if your internet connection slows, the VoIP system will have priority through the router. This should maintain the quality of the VoIP transmission and make it more likely that the transmission will be understood on the other end.

It’s important to note that there are many variables here – the VoIP provider, your internet provider, your fax machine, other systems that you are using on your network – as well as all the same that exist in the environment with each and every other fax machine you do business with is located.

Best options

The best options for this is to either:

a) get an online fax number (we recommend trying the service first, then porting your fax number over second – porting numbers can be a lengthy process)

b) keep a phone line from your local phone company just for your fax machine.

c) drop your fax entirely.