The rule
Calls are counted in whole minutes, rounded up. A call that lasts 61 seconds uses 2 minutes. A call that lasts 59 seconds uses 1 minute. A call with no connected time uses none.
Why
Minutes are how your plan and any extra packs are measured. Counting started minutes keeps it simple and the same everywhere you look.
Where to see usage
The Billing page shows your plan and what you have used. Each call also shows its duration.