This is not a sufficient details for setup the VoIP setup through asterisk. Do you wants to connect your branches over VoIP or same PSTN line your branches used for calling to each other.
As per you details you have a server and having the PRI Card (Asterisk supported) where you can terminate you PRI line and setup the SIP extensions for you office. Once you branch people call on your PRI line that call will be ringing on your SIP extensions according to asterisk configuration.