The two copper phone wires running from your house or office go to a nearby central office, a windowless building full of equipment that connects to each pair of wires.  The distance to your central office and the sort of amplification and other equipment installed between you and the central office will determine what sort of DSL service you can get.  Nearly all US central offices are equipped for ISDN, but many are still not equipped for DSL.

Note that a company that owns a central office (a local exchange carrier or LEC) is obligated to allow competitors (competitive local exchange carriers or CLECs) access to their facilities.  The payoff for the LECs is that they are allowed to enter the long distance business if they comply with the law.
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