Well, to start with; this is perfectly possible and with a little study it shouldn't be hard at all to manage with 2 people.What you need is fast internet connections at each branch office. Perhaps the best way to start is forget about the data center and choose one branch office to host the server. At the chosen office, you place one server running Asterisk and eventually OpenSER to act as a proxy for NAT traversal. The phones at other branch offices simply register with their appropriate internal extensions at the central asterisk server. You have the choice whether you'd like to connect the asterisk server to the outside world using VoIP/SIP or using POTS (copper). In the latter case, you need some kind of interface card (like the TDM40x series from Digium). Eventually, you could forget about the OpenSER thing, and use VPN connections between the buildings to avoid NAT. When doing so, you will be able to use Trixbox instead of "pure" Asterisk which will make life a little easier if you are unfamilliar with Asterisk configuration. However, when it comes down to things like securing the setup, and troubleshooting you will be better of when you actually know what's going on. Therefore I'd recommend you to read some documentation on Asterisk like Asterisk: The Future of Telephony (free download, as ebook).
Good luck, and happy VoIP'ing!