When the conversation is over, one or both parties hang up by replacing their handset on the base. This opens the switchhook and stops the flow of DC current.
The exchange then initiates the process of taking down the connection including again notifying the billing equipment if appropriate. Billing equipment may or may not be involved as calls within the local calling area may be either flat rate or message rate. The local calling area includes several nearby exchanges. In flat rate service, the subscriber is allowed an unlimited number of calls for a fixed fee each month. Message rate subscribers pay a charge for each call which depends on the distance between the calling and called parties and the duration of the call. A long distance call is a call out of the local calling area and is always billed as a message rate call.
In early telephones the current was generated by a battery. The local circuit included, in addition to a battery and a transmitter, one winding of a transformer called an induction coil; the other winding, connected to the line, stepped up the sound wave voltage.
