Public Switched Telephone Network (PSTN) technology was the first widely used telephone system, and it involved the use of copper wire to transmit analog signals for voice communication. PSTN is still in use today, although it has been largely replaced by more advanced technologies in many parts of the world.
Integrated Services Digital Network (ISDN) was developed as an upgrade to PSTN technology, and it allowed for the transmission of both voice and data over digital lines. This made it possible for people to use the phone for more than just voice communication, such as sending and receiving faxes or accessing the internet.
Voice over Internet Protocol (VoIP) technology then emerged as a way to make phone calls using the internet instead of traditional phone lines. This allowed for much lower cost long distance calls, and made it possible to make phone calls from anywhere with an internet connection.
Finally, Voice over Cloud technology refers to the use of cloud-based services to make phone calls. This allows for even greater flexibility and scalability, as it allows businesses to make and receive phone calls using cloud-based software rather than traditional hardware.