Tiers, balance and top-ups
What a tier includes each month, and what happens when a call would pass it.
Two things pay for a call, in this order.
Your tier carries an allowance that renews. Ordinary use lives here and costs nothing extra.
Your balance covers what goes past the allowance. It is money you have already added, and it is spent only when the tier's allowance would not cover the call.
At the wall
When neither would cover it, the call stops before it spends anything. You get told, not billed. Nothing is half-done and nothing is queued waiting for money.
Who pays
The caller, not the owner of the brain being asked. The exception is the widget: an anonymous visitor has no account, so the owner who installed it pays for those conversations.