Digital Subscriber Line (DSL) uses copper phone lines to transmit data and voice.
These lines are already running to your house or business, which is why telephone providers (POTS) became ISPs.
The provider will have a piece of equipment called a DSL Access Multiplexer (DSLAAM) at the local central office (CO) where your phone line is wired for dial tone.
The DSLAM is between the POTS in the CO and your house or business (premise).
The DSLAM communicates with the modem at your premise by using the frequencies above 3400 hertz.
The POTS system filters anything above 3400 hertz, which is why music sounds terrible over a phone call.
Filters are placed on the existing phones at your premise, so your calls do not interrupt data communications and your voice calls are not distrubed with the modem's creeching of data.
