Usually some type of loop test will be run before the installer comes. If the test is negative then the install will likely be cancelled and you can go back to the part about FOC dates. If your service is using line sharing then he will likely install filters as described below in the self-install section. If they determine that your wiring is bad then you might have to hire an outside contractor or pay additional fees for the wiring. An inside wiring contract with your Telco might cover this.
Apartments are special cases and also present a lot of trouble.
When the phone company comes out they are supposed to supply demarc information and tag the DSL line. Add two weeks to have the phone company come back out. Some buildings will have managed wiring, where you have to pay them to have an outside contractor to pull the DSL signal from your demarc to a jack in your apartment. In other cases a free pair may not be available to your location and would require having someone do the wiring. If you are in an office then your DSL will have to be on a dedicated line, same as with a modem or fax.