Which voice processing system is most commonly used in large private telephone systems with more than 35 telephones?

The Private Branch Exchange (PBX) system is specifically designed for managing a large number of telephone lines and internal call routing within an organization. It serves as a private telephone network that allows users to communicate with each other internally and externally through a limited number of external phone lines.

In large organizations with more than 35 telephones, a PBX system is particularly beneficial because it provides advanced features such as call transferring, voicemail, automated attendants, and the ability to support multiple calls simultaneously. The scalability and efficiency of PBX systems make them well-suited for larger private telephone systems compared to alternatives.

While Voice over Internet Protocol (VoIP) has gained popularity for enabling internet-based calling, it typically supports both larger and smaller setups. However, VoIP can be implemented as part of a PBX system, which makes it less specific to the needs presented by organizations with a large number of lines. Key systems are generally limited to smaller installations and start to lose efficiency and capability as the number of telephones increases. Digital signal processing (DSP) is more about technology applied to audio signals rather than a standalone telephone system.

Thus, the PBX remains the most widely adopted solution for managing complex and extensive private telephone systems.