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I have customer with 3 branches and I am managing network in all of them. VPN is ready in fully mesh topology. And i need to do the following:

1- allow free calls between branches (Perfer to use desktop phones better than softphones).
2- When any branch recieve calls to be able to transfer it to any other branch for free. ( in other word to transfer incoming calls over PSTN to any other branch over VoIP)
3- count of extentions ( empolyees 20-30 in each branch).
4- count of PSTN lines ( 4 lines in each branch) - plugged in old centeral ( not IP-PBX) that old centeral allow us to trasnfer from PSTN lines plugged to it - to the extentions plugged to it - (curtrent extentions are analog not over IP network)
5- will be great if we can handle incoming calls over PSTN with IVR.
6- will be great if you can help me through step by step.