Telephones have typically received the small amount of power required for their operation directly from the telephone lines. With such telephone sets, the telephone may be operable even though the primary line power to an establishment is interrupted.
More recently, microprocessor based electronics have been included in telephone sets to provide for telephone functions which are based on digital processing. For example, a central processing unit (CPU) based on a microprocessor may provide for automatic dialing from a directory of electronically stored numbers or from temporary storage of the last number dialed by the telephone set. Where the telephone set is to be used for transmitting both voice and digital data signals, the CPU may control routing of the signals to and from speakers and microphones and to and from digital processing circuitry. Where the telephone set services two or more telephone lines, the CPU may control selection of a single line or of both lines for conference calls.
The power available on the telephone line is insufficient for the digital processing circuitry of the more sophisticated telephone sets. Thus, the electronics rely on the normal local power. To prevent noise on the phone line from the power line, the circuits powered by the phone line and local power line must be isolated from each other through transformers, optical couplers and the like. The isolation is most effective and simplest when made at or close to the incoming telephone lines. As a result, substantially all of the telephone circuit is powered by the local power line. A disadvantage of such systems is that the telephone set becomes dependent on both telephone lines and power lines for operation. Simpler telephones may be operable with interruptions in power, but the sophisticated electronic telephones become inoperable.