DSL modem and FAX on the same line

I’ve got the DSL and the FAX on the same line in my growing little internal network. The problem is that it’s not working; even though it should, in theory, but the FAX doesn’t maintain a dialtone. I can hook a FAX to another outlet in a different room, but I need it to be in the same room/on the same outlet since that’s where my print server is. I assumed the problem is with the little piece-o-junk splitter that Verizon sent with the DSL modem. So I bought another at CompUSA but that one’s no better (on the other hand I have learned the difference between RJ11 and RJ12, and knowledge is always a good thing). :slight_smile:

So, is there a better highend splitter that should resolve this problem? Or am I doing something totally wrong? Please help me in my quest for more geekiness!

  • Shibb

You don’t say, but if you don’t have a DSL filter on the line for the fax machine, you need to install one. By the same token, be sure that each split in the line has one and only one DSL filter installed, except the one to the DSL modem, of course.

Is the fax on an extension? I ask because I just dealt with a very similar problem to this; the line comes into the IT room, where there’s a filtered socket with DSL and analogue outlets; the router was plugged into the DSL socket, but the analogue line was wired into the patch panel and patched out across the office on Cat5 to a socket next to the fax machine. We kept getting dropped DSL connections and loss of dial tone. Turned out that there was a wiring fault between the patch panel and the floor socket out in the office.

I know some fax machines do try to use frequencies that can upset a DSL connection though, but it seems more likely that if your filter was duff, the connection would fall over, rather than your dial tone disappear.

IANATelecoms engineer though, so take that with a modicum of salt.

To answer questions:

  1. Yes, I have a filter on the other lines, and on the line to the FAX machine, but of course no filter on the DSL line itself. FTR, the DSL works just fine and doesn’t every drop the line, as far as I’ve noticed.

  2. The line used is the third line in a hunt-and-seek (calls come in on line xxx-5051, then go to xxx-0151, then finally to xxx-0849). I need to have the Fax, on this third line so that I can direct other faxes to that number. Lines 1/2 are on the first jack, line three (0849) is on the second jack. I have outlets for these in three rooms. The building is about 25-30 years old, and the phone jacks are possibly newer than that but not by a lot, I’d reckon. That’s about as far as I followed you, Mangetout, Telecom engineer or no.

I would suggest a bit of additional testing. Try the fax completely plain jane, plugged into the desired jack with no splitter, no DSL, no filter. Then add these things back one at a time until it stops working. Only change one thing at a time, don’t change wires either unless that’s the only change.

In theory the fax shoul not be affected by the ADSL filter but, in practice, I have found that if I disconnect the ADSL modem and take the filter out the fax will mostly connect at 14400 while with the ADSL on and the filter inline with the fax it mostly connects at 9600. YMMV


I’d recommend calling out the phone company. I have my lines set up in exactly that fashion and have no problems at all. Of course thay came out and instaled new lines (my building is at least 50 years old) the phone company told me the old equipment (the are whre the lines enter the building) just wasn’t capable of handling DSL and an additional number. If I recall, it cost about $80 for the service call, took less than an hour.