VoIP (Voice over Internet Protocol) is the umbrella term for voice calls delivered over the public internet rather than over a cellular voice network. Skype was an early consumer example; Quo, Grasshopper, Google Voice, and Dialpad are business examples.
VoIP business phone services deliver a phone number as a software application — calls run over Wi-Fi or cellular data, ring inside the app, and require the app's listener process to be alive on the device. Audio quality is bound to the IP route in the moment, which is why VoIP can sound great on a fast home network and degrade on bad airport Wi-Fi.
VoIP unlocked features (programmability, deep CRM integration, contact-center analytics) that legacy phone systems couldn't deliver. The trade-offs — Wi-Fi dependency, app-must-be-open, audio quality bound to bandwidth, "virtual number" 2FA blocking — are the failure modes that business eSIM products like Kelir exist to remove.