Some prepaid phone providers have the following system in place:
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Users have pay a certain fixed activation fee upfront, however this amount will be converted into tokens that go towards their initial credit balance (expressed as the remaining number of tokens).
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After the initial activation, the users are able to top-up their own credit balance at any time. It is possible to buy different tiers of prepaid credit, each available for a fixed fee ($1, $5, $10, whatever, …). Of course, the price of each tier corresponds to an equivalent number of tokens that will be added to their credit balance.
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If users are low on credits, they can either manually top-up with a one-time payment or they can agree to be automatically charged after their credit reaches below a certain threshold.
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All purchased token credits are only valid for a fixed period (e.g. 6 months) after the purchase date All unused token credits will expire after this time. Of course, older credits will be consumed before any newer credits are consumed.
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You could differentiate your product by limiting some features to users who have bought more expensive tiers. Users on the prepaid plan however will need to top-up that minimum amount at least once every month (meaning: they need to top-up again within a time period of maximum 31 days following the previous top-up) to keep these functionalities unlocked.