From all we said before we noticed that we still have not solved problems about bandwidth, how to create a real time streaming of data.
We know we couldn't find a solution unless we enable a right real-time manager protocol in each router we cross, so what do we can do?
First we try to use a very (as more as possible) high rate compression algorithms (like LPC10 which only consumes a 2.5 kbps bandwidth, about 313 bytes/s).
Then we starts classify our packets, in TOS field, with the most high priority level, so every router help us having urgently.
Important: all that is not sufficient to guarantee our conversation would always be ok, but without an great infrastructure managing shaping, bandwidth reservation and so on, it is not possible to do it, TCP/IP is not a real time protocol.
A possible solution could be starts with little WAN at guaranteed bandwidth and get larger step by step.
We finally have to notice a thing: also the so called guaranteed services like PSTN line could not manage all clients they have: for example a GSM call is not able to manage more that some hundred or some thousand of clients.
Anyway for a starting service, limited to few users, VoIP can be a valid alternative to classic PSTN service.