We have several GrandStream286 ATAs on our office LAN and they are able to access the 3CX IP PBX server. They are all configured to access the internal IP address (192.168.x.x) of the server. All of the phones work fine. However, when we configured several of the ATA to access the server's public IP, so that we could access the server from the WAN (for employees working from home or on the road), the phones are not able to register. Is there some special feature that has to be enabled in the GrandStream286 or the 3CX server's configuration?