The voltage supplied is a compromise between operational needs for reliable service and safety precautions for customers and service personnel. The length of the line to a customer telephone interface presents a resistance across which the central office voltage experiences a drop and therefore the voltage at the customer site may vary.
To ensure reliable service, the voltage supplied balances operational requirements with safety considerations for both customers and staff. As this voltage travels through the line connecting the central office to the customer's telephone, it encounters resistance, leading to potential fluctuations in the voltage at the customer’s location.