Correct Router Configuration is a key element in ensuring correct communications between 3CX PhoneSystem and external entities such as external extensions, VoIP Provider services, and other Bridged PBXs. NAT and/or PAT When a network host A sends a request to another network host B, it will be expecting a reply to the request. When both network hosts reside on a Public IP Address, the exchange is direct, and simple enough. If one of the network hosts resides on a Private IP Address, however, things start to be come more complex, because the network environment is a NAT/PAT environment. If network host A is in a NAT/PAT environment, when it sends a request to network host B, it will build its request using the only IP Address it knows – its private IP Address

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Router Configuration for 3CX PhoneSystem