Information on hooking up a fax machine
POTS

Here's a decent one-page reference, where you are probably only interested in the first paragraph and diagram about the RJ11 plug: http://www.westernet.net/Help/RJ45.htm



Traditional telphone (POTS="Plain Old Telephone Service") uses "quad" wiring to support up to 2 telephone lines. That's the traditional telephone cable with black, red, green, and yellow wires. If you're looking at the wires in a RJ (registered jack) connector, they belong in that order. The mnemonic "BuRGundY" may be helpful. Each line uses one pair (2 wires) for a voice or fax connection.

Line one uses the red and green wires. Line 2 uses the black and yellow wires.

A single-line configuration (red/green) uses the two pins in the center of the connectors.

In a two-line configuration, the second line (black/yellow) uses the two pins to the outside of the center two pins.

DSL

DSL (ADSL, VDSL, etc.) also uses two wires (one pair), like POTS. Both DSL and POTS signalling can be transmitted on the same wires because they operate at different frequencies. But the DSL signalling causes interference with analog sound signals and needs to be filtered out. You use a DSL splitter to do this. 

The DSL Modem

Your specific DSL modem might have an internal filter and might thus provide a clean analog phone jack. If not, then you need an external splitter.

If you do need an external splitter, then you need to plug the splitter into the wall, and plug the DSL modem and any analog telephony devices into their respective jacks on the splitter.

The Phone Company

You also need to have POTS service on your line from the phone company. Just because you have DSL service doesn't mean you have analog telephone service.

I don't have analog phone service from the phone company, for example. I do have a "home phone" line that I can use for the rare fax transmission, but it is connected to a VoIP device that is connected to my Ethernet network.