Connecting 10 Remote Offices including 10 Grandstream  IP PBX with the HQ Main Grandstream PBX Over IP VPN & Connecting the Remote offices ( 150 Branches ) with the HQ  using OPENVPN Between the IP Phones & Central PBX . Professional Services including Call Routing between the Remote branches , High Availability using two ISP with Active / Active Failover , Load-balancing on external / PSTN Lines to decrease the Phone call Costs