Voice over Internet Protocol (VoIP) is essentially a telephone connection over the Internet. The data is sent digitally, using the Internet Protocol (IP) instead of analog telephone lines. This allows people to talk to one another long-distance and around the world without having to pay long distance or international phone charges. This can be done through various applications, such as Skype, WhatsApp, Google Hangouts, Zoom, and Facebook Messenger.
Source: TechTerms, Vonage
Additional Reading: RingCentral Offers 3 Free Months of VoIP to Organizations Impacted by Coronavirus