Which of the following is a characteristic of direct line telephones?

A direct line telephone is specifically designed to connect two predetermined points, creating a dedicated line for communication. This characteristic is essential for ensuring secure and consistent communication, often used in situations where reliability and clarity are paramount, such as in emergency response scenarios.

The other options represent features not typically associated with direct line telephones. Multi-line systems are more common in office environments to handle multiple calls simultaneously, cordless operation refers to phones that do not require a physical connection to a landline for operation, and high-speed internet support pertains to data transfer capabilities found in different technology infrastructures, not in direct line telephony.