Long Distance is different than other services because other services have a predefined charge that you pay every month, while long distance almost always depends on your usage. For example, you pay the same every month for your Internet service, but you use a different number of minutes every month for your long distance phone calls. This means that we cannot bill 1 month in advance, because we do not know what that amount would be. This means that you see charges for your long distance usage from the previous month, on your regular monthly invoice.
In some cases, your long distance plan has a monthly amount that you pay, for a specified number of minutes, and you only pay for usage over the predefined amount in your plan. In that instance, you are billed one month in advance for your regular monthly charges, but your over usage is billed for the previous month that just past.