BuddyPress.org

BuddyPress forums are group based by default, so you could set each group up to be an office and each office would get its own forum. It would all come together on the forums directory. From what you wrote it doesn’t sound like you’ll be needing all the social networking that ships with BP, so WP MS (setting offices up as blogs) and a standalone forum (without BP) might be better suited to your needs.