A common approach is to divide the input domain into Equivalence Classes such that the program can reasonably be expected to behave the same for any points within a class. On dial direct calls, you pay only for the minutes you talk. The initial rate period is one minute.
Additional savings apply evenings, nights, and weekends.
Whenever operator assistance is required or you use your calling card, there is a service charge in addition to the low direct dial rate. A transaction is a unit of work as seen from the user's point of view - it consists of a series of tasks which may or may not be visible to the user. Tasks are activities that the user or system does - Sometimes in the process of a transaction, you may have a choice of several tasks, and depending upon what choice you make, the tasks to be performed may change. This is especially true of modern,windows based applications. The task to be performed is usually preceeded by one task, and followed by another.