How much does that cost?
In the UK our air-time credit (top-up) is in the form of money. For example, you could add £5 to your account. Calls are then charged per minute, per text , per picture message, per Kb of data.
The top-up doesn't expire and there is also no service element, we don't pay for incoming calls either.
Although, you can top up by entering a code on your handset. Many people, top-up using a swipe card, which can be used in tens of thousands of shops. You just hand your card over with cash and the top-up is applied in a few minutes. Other people, register their debit or credit card and can top-up by entering the card details.
The networks also sell bolt-ons which are bundles of minutes/texts/data at a reduced rate. For example, O2 sell a months data for £7-50.
From what I've seen, we pay less for our calls than pre-pay customers in the US and much much less for data. The phones also have more features.