Based on consultant advice your company has migrated from PSTN network to VoIP network.?

Gateway Protocols: Based on consultant advice your company has migrated from PSTN network to VoIP network. You have replaced PBXs switches with Cisco call managers, og phones with IP phones and routers with Cisco voice enabled routers. Now you are in process of selecting gateway protocol and signaling protocols. Which protocol would you select for your voice network and why?