What is a common requirement when checking telephone line voltage?

When checking telephone line voltage, it is essential to disconnect the line from any devices to obtain an accurate reading. This is because devices connected to the line may draw power or alter the voltage readings due to their operational state, thereby providing misleading results. By disconnecting the line, you eliminate the influence of any connected devices, allowing you to measure the true voltage present on the line itself.

Other options, such as ensuring the equipment is powered on or that the right connectors are used, do not address the primary concern of accurately measuring line voltage. Likewise, powering down all devices may not be necessary if only one device is influencing the measurement, meaning disconnecting the line provides a clearer and more specific approach to accessing the voltage levels.