When two tones are used to signal switching equipment, what is this signaling method called?

The correct answer is DTMF, which stands for Dual-Tone Multi-Frequency signaling. This method uses pairs of audio frequencies to represent each digit on a touch-tone keypad. When a key is pressed, it generates a combination of two distinct tones, hence the name dual-tone. DTMF is widely used in telecommunication for signaling and managing phone systems, specifically to communicate with switching equipment to set up calls or access services.

In contrast, the other options refer to different communication technologies or systems. Centrex is a type of telephone service offered by a telephone company that provides some exchange functionality directly on the subscriber’s premises without the need for traditional PBX hardware. A Key System is a simpler type of business phone system that allows multiple telephones to access multiple lines, providing a straightforward way to manage calls within an organization. A Tie Line refers to a dedicated circuit that connects two or more telephone systems or private branch exchanges (PBXs), allowing them to communicate directly with each other.