FoIP over LAN

i searched a lot the web for what i need with no results, so i try asking here. :smiley:

My question is simple: is it possible (and how) to create a “local” voip/foip service over a private network?
I have what could be called a private MAN (metropolitan area network) that connects about 10 offices. In all offices there is an ethernet LAN already linked to the MAN through a packet commutator (don’t ask me details, anyway should not be necessary). My goal is to install in every office a fax/phone over IP so that i can phone call and fax from an office to another without using internet or phone line, just the LAN/MAN. There’s also a server room that manage the whole MAN.
Is asterisk software able to implement this? Any particular hardware to buy?

Thanks for any help