Lumen help

Using a fax machine with VoIP

A fax machine is basically an image scanner, a modem, and a computer printer combined into one device that converts optical images into electrical signals to transmit over communication systems (phone line, internet, etc.). When the fax machine receives the electrical signal, it converts the signal back to an optical format to display or print the original image. This process is called facsimile transmission or faxing.


In the fax relay technique (not supported by Lumen) a faxed call over VoIP uses the VoIP device, generally called an analog termination adapter (ATA), to spoof a fax machine on each end of the relay.

Note: Not all types of fax machines work with VoIP.

Here are some things you should check to help your fax machine work with VoIP:

  1. Check below to see if the fax machine is compatible with VoIP and get suggestions for configuring your machine if it is compatible:
Fax machine type VoIP compatible? Configuration
Group 1 (G1) and group 2 (G2) fax machines G1—no; G2—can interoperate with G3 machines. No recommendation—G1 and G2 fax machines are obsolete.
Group 3 (G3) Yes, however, if your G3 fax machine includes super G3, you’ll need to turn it off. If super G3 is on, you transmit faster, and that’s a problem for VoIP.
  1. Set (or reset) the modem speed to 14.4Kbps.

  2.  If your fax machine uses super G3, turn it off.

Note: You may need to refer to the fax machine manual or check with the manufacturer to find out how to turn off super G3.

Super group 3 (super G3) No, in most cases. Super G3 fax machines don’t work well with VoIP, but you can turn off super G3 and try faxing.
  1. Turn off super G3 if possible. (Refer to the fax machine manual or check with the manufacturer to find out if you can turn off super G3 functionality.)

  2. If super G3 is turned off, set the modem speed to 14.4Kbps.

Note: CenturyLink cannot help you turn off super G3 on your fax machine.

Group 4 (G4) No, Group 4 fax machines are specifically designed to transmit on 64Kbps ISDN circuits and do not work with VoIP service. Not applicable.

Note: Many fax machines offer super G3. Lumen has tested Brother 8220 and 8500 fax machines with VoIP service and they are compatible. Other Brother fax machines have not been tested on Lumen VoIP.

  1. Use the fax pass-through technique for using your fax machine over VoIP. Lumen supports the fax pass-through technique, which involves a sending machine and receiving machine to communicate directly. The sending machine calls the receiving machine and the call is converted to VoIP during the transmission.