Actiontec DSL modem/router - Transparent Bridging mode
If you are placing a Protel PBX behind a NAT router that is connecting to the internet via a DSL connection, you will need to disable the NAT functions of the DSL modem/router. If NAT is not disabled, you will very likely not be able to register remote extensions, especially if the remote endpoint is connecting from a dynamic host address - a softphone app running on a smartphone that is connecting via the cellular data network, for example.
The specific configuration described further down here is for an ActionTec C1000A - the DSL Modem/Router combo device provided by the ISP - and a Ubiquiti EdgeMAX router. The hardware may vary in other scenarios, but the underlying concept is the same regardless of the configuration, and it applies to other types of ISP connections as well: If you aren't going to use the router functions that are included in nearly every ISP-supplied modem (and you shouldn't!), you have to make certain that the ISP device is set to 100% passive bridge mode.
Log in to the modem’s WebUI (usually located at 192.168.0.1)
Under “Modem Status” (orange arrow) there is a section called “NAT Table” (red arrow). That will let you see if the modem is playing with your port mappings.
To bypass the modem and its NAT, you’ll need to put the modem in “Transparent Bridging” mode.
On our CenturyLink C1000T, this was in “Advanced Setup” (see the blue arrow above) à “WAN Settings”
The settings that are necessary here vary depending on the accounts and the lines in a customer’s area, but for us, we chose “Transparent Bridging Untagged.”
Before clicking apply, the customer will need to know their PPPoE/PPPoA details or Static IP (again, depending on their specific setup), and have their router set to negotiate those on the modem’s behalf.
We set the Internet connection type to PPPoE and plugged in our account details, clicked apply on the modem, clicked apply on the router, and we were in business.