CenturyLink DSL is much more sensitive to distance from the network interface and wiring situation. If they say they can't get you the speed you want, they can't. Go elsewhere if you can.
In my neighborhood, CenturyLink can deliver an honest 40 megabits down, 20 megabits up. By honest, I mean that Cox's 55 megabits download speeds are actually slower in the real world.