What you need is to setup some “port forwarding” rules on your AirPort. Actually its name in the AirPort Utility is “Port Mapping”, under the “NAT” tab of the “Internet” section.
First you need to check your ports on the Grandstream HT702. If you go to the Web interface of the Grandstream HT702, under the “FXS Port 1” and “FXS Port 2” tabs you can find the configured ports for each “phone port”. The fields are called “Local SIP Port” and “Local RTP Port”. Take note of those ports; usually 5060 and 5004 for “FXS Port 1”, and 6060 and 6004 for “FXS Port 2”. It is very important to change the field of “Use random ports” to “No” on both tabs and apply both changes, so our next step is not in vain.
Now with the ports information you proceed to “Configure Port Mappings…” as mentioned before in the AirPort Utility application. In there you want to click on “+” to add a “port mapping rule”. In the “Port Mapping Setup Assistant” window input the WAN IP address of the Grandstream HT702 in the field “Private IP Address” and all the ports, “5060, 5004, 6060, 6004” in the fields: Public UDP Ports, Public TCP Ports, Private UDP Ports and Private TCP Ports. Click on “Continue” and name your service “VoIP” or whatever you prefer. Apply all these changes and remember to uncheck the “Enable default host at:” under the “NAT” tab.
Restart your Airport and later your Grandstream HT702.
Now you are ready to receive and make calls.
In short, the ports that you just open in your firewall are for receiving and making calls (the 5060 for line 1, and 6060 for line 2) and 5004 and 6004 for the actual audio of the calls for line 1 and 2.
Now enjoy your Grandstream HT702 and your new VoIP experience.
Thank you. This helped. But, there is an additional step that was necessary with the AirPort Utility application. Press 'Internet' button; select 'Internet Connection' tab; select 'Connection Sharing' parameter, and change it to 'Off (Bridge Mode)'. Only when this step was added did it begin to work. It was the AirPort Utility application itself which suggested this additional step.