The PSTN, or Public Switched Telephone Network, is a circuit-switched telephone network that has been in use for nearly 150 years. It works by using underground copper wires to connect homes and businesses to switching centers, where calls are connected to each other. The process involves dialing a number, which is routed through the PSTN to the recipient's phone. The PSTN uses traditional methods such as area codes, exchanges, and toll offices to route calls, but has been largely surpassed by more modern communication solutions like VoIP. Businesses often use private branch exchange (PBX) systems or hosted PBX solutions, which offer features like auto-attendants and scalability, making the PSTN less necessary for their needs.