If an organization already has an old PBX in place, they may consider it too expensive to migrate the whole organization to new Voice over IPPBX technology.
In this case, a mid level solution can be combining the old PBX and building a private network between all the branches in the company. This solution is cheaper than VoIPPBX providing the same function: All the international calls can be sent through a Voice over IP (VoIP)
The whole organisation will be able to talk for free between branches, even if in different countries.
Calls out of branch will be the cost of a local call.