Telephone service at the customer end hasn't really changed much since the 20's. The media has improved but the basic POTS is regulated to the point that it can't change. Al Capone's candlestick phone will connect and dial on a DSL line.
The central office changed from several floors of clicking relays to a small box humming in the corner and the trunks are digital fiber but the last hundred feet from the concentrator to the house is still a copper pair with a 10-48v analog signal on it. The unbundling of "Ma Bell" and the rise of consumer owned equipment created the Demark box on the outside that has a RJ11 plug in it so you can isolate the "house" side from the Telco side for troubleshooting.
If a phone works in the Demark, it isn't the Telco's problem and they will charge you a stiff service call fee (~$135-150US)if they come out.
