There are a number of ways you can implement the 3CX system, including on the local office LAN, or in a data center. While its recommended use is in on the local LAN, with the implementation of VPN the data center would be a better solution if you need to service multiple offices in geographically diverse locations. As I talked about in this topic,http://www.3cx.com/forums/3cx-system-on-hosted-server-11039.html, there are a number of providers out there who offer a hosted solutions and are quite successful at it. It really comes down to what you need out of the system, and how much money your willing to invest into infrastructure.
3CX is SIP compliant, So long as the provider/client supports SIP, location doesn't matter. You can setup different SIP trunks in different countries and setup a dial plan to use that to its fullest extent.