You may also have interior wiring issues. Here's how I fix inside wiring for DSL accounts.
Purchase a DSL splitter and install it at the telco service entrance. Run a direct line from the DSL port on the splitter directly to the DSL modem. Connect all of the other phone instruments to the telephone output of the DSL splitter. All of the DSL analog side wiring should be CAT3 or better twisted pair from the telco service entrance to the DSL/ADSL modem. You can use one of the twisted pairs in CAT5 cable if you have that on hand.
This is as good as it gets for DSL installations, and will usually solve in-house wiring issues.
Here's a good detailed description of the process:
DSL Wiring Upgrade Tutorial