This is probably more than you want to know, but it shows how simple mistakes can screw you up:

My first DSL had a dedicated line, with no phone equipment connected at all, and a hard-wired wall plug. The new DSL connection needed to come in over one of my existing phone lines to get the low price. I picked my Line 2 which is normally used just as a fax connection. I have a wall plug with two outlets, one disconnected, the other carrying both lines for my two-line phone. I long ago had popped a 3-way splitter on the working plug, which has three outlets: Line 1, Line 2, and Line 1+2. Line 2 goes to the fax, and Line 1+2 to my two-line telephone. Now, I could have stuck another 2-way splitter on the Line 2 outlet so I could connect both my fax and my DSL bridge, but I worried about too many cheap connectors stacked together.

So, I decided to just rewire the wall plug to hook up the unused outlet to Line 2. I did this, and couldn't make the DSL connection. I spent two days trying to figure it out, including a couple of fruitless support calls, until I was sitting there pondering my 3-way splitter and noticed that Line 1 and Line 2 plugs both had only the center two Line 1 connectors in them, i.e. red and green

I had wired the wall plug to put Line 2 on the Line 2 connectors (the outer two on a 4-wire phone plug, or yellow and black). I rewired the orange Line 2 wires from the wall to the Line 1 contacts in the outlet, and my modem lit up when plugged in. Huzzah!

It took me TWO DAYS to catch that. Want to know how I was testing the new connection before I figured this all out? Hooked up my 2-line phone to it, punched up Line 2, and zing! Perfect dial tone! So what's the problem with my damn modem?

Gah.

Hooking up a normal single-line phone would have given me dead air, and I might have gotten a clue.