There are very few advantages of using a modem to fax from a Mac nowadays but one of them is that it doesn’t require an internet connection.
It does however require a phone line as you need to plug the USB modem into your analog landline phone line to send and receive faxes from your Mac.
Unfortunately, since macOS El Capitan Apple has removed legacy modem support from macOS.
This means that in the latest versions of macOS Sierra, High Sierra, Mojave, Catalina, Big Sur, Monterey and Ventura you can’t use an external modem to fax from a Mac.
The good news however is that it’s still possible to install El Capitan on a separate partition on your hard drive so that you can connect an external modem to your Mac and use the El Capitan fax utility.
Note that this is the only way to connect a modem to your Mac. You can’t fax from a Mac using an iPhone or iPad as a tethered modem.
Firstly you’ll need to install El Capitan on a separate partition on your Mac.
As long as you’ve got an Intel Mac (not one of the new M1/M2 Macs which do not support Boot Camp) you can do this using Boot Camp which is a free utility in macOS and allows you to install other operating systems on your Mac.
Although it has been discontinued and you may be able to buy second-hand ones online, do not buy the Apple external modem as it only works with OS X 10.6 Snow Leopard and below and is only compatible with G5 iMacs, Mac minis or G5 Power Macs although nowadays.
3. Add The Mac Fax Modem
Plug in the modem to a USB port on your Mac.
Go to Apple menu > System Preferences, and then click Printers & Scanners.
You will see the modem appear under the list of devices.
If you don’t see it, click the “+” symbol and select your USB modem from the pop-up dialog and click Add.