Several ways to go here I don't really recommend the VOIP over the internet but your going to have to do a user count and peak call utilzation survey for each site.
Your probably going to go with one of the ASxxxx models for the central site and 2800 or 1800 for the remote sites. As for the VPN offices you might try CME Call Manager Express for them or keep there existing PBX's kinda depends on what there Goals are.
I can't give you specific parts because there is such a huge combination of different ways to go but your gist is you'll probably have a couple of Access-Servers to handle your primary site PSTN connections and then put something that can handle SRST Survivable Remote Site Telephony at each branch office. For your VPN sites your going to want to use Call Manager Express.
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