Request to add a dual prepaid/post paid account type

Let's say I want to offer a prepaid package deal which offers 'x' amount of calling per month in a certain country.

Any calls outside that area would be considered “long distance” thus post paid and billed at the end of the monthly billing. period.

So at the next billing, the amount due will be the price of the prepaid product plus any "long distance" calls made in the monthly period.