What function allows interoperability with traditional phone systems?

The function that enables interoperability with traditional phone systems is Session Initiation Protocol (SIP) Trunking. SIP Trunking serves as a bridge between Voice over Internet Protocol (VoIP) systems and traditional Public Switched Telephone Networks (PSTN). It allows voice and multimedia sessions to be managed over the internet, effectively connecting VoIP-based systems to external telephone networks.

This technology facilitates the transmission of voice communications, enabling businesses to make and receive calls to and from conventional telephones, while taking advantage of the benefits that come with VoIP, such as lower costs and added functionality.

In contrast, other options like Virtual Private Network (VPN) are designed to create secure connections over the internet but do not specifically address interoperability with phone systems. Voice over Internet Protocol (VoIP) refers to the technology that allows voice calls to be made using the internet but does not imply the interoperability with traditional telephone systems without a mechanism like SIP Trunking. Internet Protocol Private Branch Exchange (IP PBX) is an advanced telephony system that manages internal and external calls within an organization, but it also does not inherently ensure connectivity with traditional phone networks without additional components, such as SIP Trunking.