VoIP

PSTN is the Public Switched Telephone Network, the system
based on circuit-switched technology for real time voice
transmissions.

The term PABX refers to a Private Automated Branch Exchange.
It is a business telephone network where calls are digitally routed within
the organization.

VoIP stands for Voice over Internet Protocol. An Internet telephony system uses hardware and software to send voice data packets over the Internet.

With VoIP telephones, adapters and equipment, low-cost phone calls are transmitted digitally over computer networks.