Maybe if you have REALLY shitty phone lines. When I first got my DSL modem I used this old ass 50' length of cable to test it out because the jack was too far away...and I got like 30kB/sec download maximum. Once I got some new cable and wired it up the right way, my downloads maxed out at what they should have.
Somewhere outside there is a grey box located on the side of your house.
That grey box is the point where the phone company is responsible for lines, between the box and the telephone pole the phone company owns, between the box and the house you are responsible for.
If you open the box up you should see an open phone jack (is marked test or some shit) go ahead and plug your DSL modem into that.
That is the best signal you can get off that phone line.
If you notice a difference in the connection it is possible to get someone (yourself, a techie friend, the phone company) to run a dedicated line from that box directly to your DSL modem/router.
I highly recomend this in houses with old shitty wiring.
Funny the question of line quality should come up.
At the house I grew up in (still in the family), the cable internet worked like shit for about a year before we realized the problem was THE CABLE RAN FROM THE BOX TO THE HOUSE ABOVEGROUND, LAYING ON THE FLOOR OF THE WOODS. Astoundingly, something (only one thing in ten years!) had nicked the casing and let water in, and about two feet of the shielding had rusted through. The cable company replaced the entire run; this time we laid down about 250 feet of ABS pipe on the ground for them to run it through, so at least the cable wouldn't get crushed by falling shit.
Just a funny story, nothing really to do with DSL.