Which technology is used for transmitting voice calls over the internet?

The correct answer identifies VoIP, which stands for Voice over Internet Protocol. This technology explicitly refers to the method of transmitting voice calls using the internet, enabling users to communicate using traditional phone services but over a digital network instead. VoIP converts voice into digital packets and sends them over the internet, leveraging existing data networks, which often leads to reduced costs and greater flexibility compared to traditional telephony methods.

SIP, while associated with VoIP, primarily serves as a signaling protocol that establishes, manages, and terminates voice calls. It is a mechanism used within VoIP systems rather than the overarching technology for voice transmission itself. Therefore, although SIP plays a crucial role in setting up VoIP communications, it does not encapsulate the entirety of voice call transmission over the internet.

ISDN, or Integrated Services Digital Network, is a circuit-switched telephone network system that enables the transmission of voice and data over traditional phone lines. However, it does not utilize the internet for voice calls, unlike VoIP, which is specific to internet-based communication.

X.25 is an older packet-switched network protocol mainly used for connecting different networks and is not specifically designed for voice communication, particularly not over the internet.

Thus, VoIP is the most accurate choice