check on something called hairpinning on your router. some routers would not allow loopback so internal members having their address mapped to the same public IP would not be able to work.
An easy way to check is setup a web server in your LAN and try to access this webserver from any machine in your LAN using the public IP of your router instead of the private LAN address. If it works then your router will allow loopback.
This is just one scenario that would stop internal calling. There are many other catchups.
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