Which system is primarily used for Internet-based calling?

The primary system used for Internet-based calling is Voice over Internet Protocol (VoIP). VoIP technology allows users to make voice calls using the internet instead of conventional telephone lines. It converts voice into digital signals that can be transmitted over the internet, enabling lower-cost and often higher-quality calls compared to traditional telephone systems.

This technology facilitates a wide range of communication options, including voice calls, video calls, and messaging, all over internet connectivity. VoIP has revolutionized how we think about calling by allowing for greater flexibility and varied features such as call forwarding, conferencing, and voicemail integration.

In contrast, the other options represent different communication technologies. Integrated Services Digital Network (ISDN) is primarily a circuit-switched telephone network system that provides high-quality voice and data transmission but is not specifically designed for internet-based calling. Public Switched Telephone Network (PSTN) refers to the traditional phone service that operates over copper wires and is not reliant on internet connectivity. CDMA (Code Division Multiple Access) is a technology primarily used in mobile network communications rather than internet-based calling. Thus, VoIP stands out as the correct choice for internet calling due to its design and functionality.