If I'm right and 100 meters is the max, you'd be over on that aspect for starters. Every foot of distance adds a nanosecond to delay time; that may or might not be much of a problem. An extra 100 feet would be .1 milliseconds which wouldn't seem like that big a deal, but then I'm no expert. Then there's the shielding to consider. Usually phone wires aren't shielded, so you could have a noise and crosstalk problem I'd think. Best case you wouldn't get anywhere near gigabit rates. With 6 pair of wires you're OK on that count, but you'd need to be absolutely sure you got them matched up correctly. Hopefully the colors would be distinct enough so that wouldn't be a problem. It certainly wouldn't be that expensive to get a couple of connector plugs and a crimping tool to give it a try it. If you do, set the transfer rate to 10 mbps to start and see if you could get that to work. Obviously you'd be on your own with this experiment, but it certainly would be the cheapest alternative.
If you do try it, I for one would be most interested to hear the results.
I would like to extend my ethernet network to my shop which is located approximately 400 feet from my house. Burying new cable would be difficult due to paved driveways and other obstructions but I have an existing unused phone line buried to the shop. This is (presumably) a high quality cable installed by the phone company to provide service to my home and is no longer used so I am wondering if I can convert it to use as a network cable? I believe the cable has 6 pairs. My intention would be to install an access point in the shop to provide wifi for my smartphone and laptop.