To understand this point it is necessary to clarify, at least in principle, how the traditional telephone network (PSTN) and the internet work.
The two networks can be thought of as two plans which have different communication technologies and, consequently, different tariff plans. On the telephone network, switching is referred to as "circuit switching", in that communication between two users is made possible by reserving a "path" that connects them during the communication. This type of use of the network has many deficiencies. The first is the fact that the network administrator (Telecom in Italy) has to build many "lanes" to ensure the accessibility of the service to a sufficient number of users. The immediate consequence of this is the high cost of connection. Traditional telephone traffic is charged on the basis of time.
Moreover, another weak point of the traditional analogue network is that the further away the users are, the busier the network becomes. And so tariffs also depend on the actual distance between the users (therefore, with the exception of promotional offers, the charges for local calls are much less than for international ones).
We could say that, by and large, the traditional network offers an on-off type of operation: either the network works, or the call is not possible because the network is blocked. The user only pays for calls that get through. In short, calls are paid for only on the basis of how many are made, how long they last and how far we are from the person we are talking to. On the other hand, attempts to connect to the network that give a busy signal, due to network congestion or a user engaged in a call, are not counted.