Circuit vs packet switching
Why does a WhatsApp call cost you nothing, while a normal mobile call eats your prepaid credit by the minute? The answer is not generosity. It is that the two calls are built on completely different ideas.
The private lane
A normal mobile call is circuit-switched. The moment it connects, the network reserves a dedicated slice of capacity just for that call, held open from the second you connect to the second you hang up. Nobody else can use that slice while you talk.
You are renting a private lane. It is yours alone, guaranteed, for the whole call. That guarantee is exactly what your carrier is charging you for, per minute.
The shared road
A WhatsApp call, or any VoIP call, is packet-switched. Your voice is chopped into thousands of tiny packets and dropped onto the public internet, mixed in with everyone's videos, downloads and messages. No lane is reserved. Each packet just finds its own way across whatever path is open that millisecond.
No dedicated lane to rent means nothing extra to bill. The call rides the internet connection you already pay for.
See both types side by side, animated
Watch a circuit-switched call hold its private line open, next to a packet-switched call scattering packets across a shared path, in a live animated diagram.
Open VoIP vs PSTN →Why this trade-off matters
Circuit switching: rent a guaranteed private lane, pay per minute, rock solid quality.
Packet switching: share the public road, pay for the road once, accept that quality depends on traffic.
That single trade, a guarantee versus shared best effort, is why internet calls got so cheap, and why they occasionally get choppy when everyone in the house is also streaming video.