iPhones cannot be used on any prepaid carrier, they have a specific contract with their respective carriers. as for other smart phones you can use them on a prepaid plan but if you want the internet to work you must purchase an extra data plan monthly. usually an additional $25 per month. it is typically cheaper to get a contract and you get the benefit of the full network, prepaid accounts do not have access to the full data or voice network.
It is not possible to buy minutes for an iPhone or a Smartphone. You only get to buy a month plan, which includes the minutes. You can also choose a prepaid carrier, or phone network, and then use that carrier's prepaid airtime card together with the carrier's phone.