The POTs (plain old telephones) were (in Germany) supplied with 60V DC by the telephone company. Each telephone exchange in Germany had an emergency power supply for this purpose. That's why the entire telephone system in Germany still was functioning even in a total blackout.
These times are over. It's now all Internet based VoIP which takes a router in your house which you have to supply with electricity on your own. As a side effect, you now pay for the power which was supplied by the Telephone Company before.
And emergency calls (police, firefighters, emergency doctor ...) don't work any more in case of a blackout (and mobile phones will fail at the latest after 20 minutes).
BTW: Tough the picture above shows a typical POT, it's not one of the legendary bakelite W48s. Those were a typical German piece of hardware, first produced as the W38 (launched in 1938) by the Deutsche Reichspost (developed by Siemens & Halske) and later updated to the W48 (in 1948) and produced by miscellaneous companies in license of Deutsche Bundespost.
Up the the liberalization of the telecommunications market the W48 have always been property of the Deutsche Bundespost and thus property of the Federal Republic of Germany. So all W48 have always been rental equipment and no such device was ever sold, nor was it ever installed offiicially in countries other than Germany.
Sorry for the off-topic, but the phone crazies in Germany would go nuts without a clarification of the origin of the "holy grail" W48 when they read this thread ;-)