This is a typical VoIP implementation utilised by larger enterprises.

 A large implementation would be where a business has multiple sites of a reasonable size, and it is more efficient and sensible to install multiple servers (usually one per main site). There is no difficulty if different sites have different PSTN connectivity, as the appropriate hardware for the connectivity at that specific site is installed in the server.

Additionally, incoming calls at one site can be easily redirected to any extension within the network.

Note: There needs to be excellent planning for this type implementation, especially regarding allocation of extension prefixes and ranges at each server, server inter-connectivity and outgoing call routing.