If you have access to your outside test jack (where the main line from your telco connects to your home line, usually via a standard sized telephone jack), find a long phone cord and run it from your DSL modem to the outside test jack directly. This bypasses all of your internal circuitry.
If you were unable to connect before and are still unable to connect, your ISP probably has a problem (perhaps a bridged tap or numerous other line quality destroyers). Call your DSL service provider or phone company and ask them to check.
If you were unable to connect before due and are able to connect after doing this, the problem is likely inside your house. Your telco probably offers professional installation of a DSL line splitter, which would eliminate internal problems. (I will post information later about self-installing a separate DSL line on the same phone number).
Also read About DSL for lots more information