Mains power keeps them charged and direct current is drawn from them to operate the exchange. The exchange also supplies direct current to all the telephones it serves. In a power blackout, while other services stop, telephones operate as usual. The reason is that batteries supply the current for the exchange until they are discharged. Most large exchanges have automatic generators which, triggered by a mains supply failure, keep the exchange batteries charged.
Rental covers the cost of maintenance - of the telephone, the exchange and the network.
Call charges mean that the heaviest users of the system pay the most. To calculate this second component of the bill, all telephone calls are metered. The meters are small and resemble water meters. For local calls, the meter clicks only once at the end of the call. For the most expensive IDD calls, the meter clicks every few seconds.