Hi,
Propose a call manager at central site with appropriate haradware and licenses depending upon the no of users.
Propose CME for the branches or you if there are a few users let the users from brnches register at the head office and use SRST.
if u can provide the exac no of users and type of connection b/w sites ;that would be better.
I mean if the connection is high speed and reliable or not .
We normally propose CME to our customers as we donot have reliable internet connections here.
HTH
regards
Nouman Khan