[Rob]

As Zeph says the quality of your phone line wiring is important. Additionally you need to live fairly close to teh BT exchange. Finally for the higher speed services you have to be in an area where the ISP has installed it's own LLU kit, and that can mean your phone call costs then also get billed via your ISP. Even if all other factors are good, you are still on a contended (shared) service and thus your actual speeds can be affected by your co-users activities.