Start by selecting the SIP tab. Pretty near the top, under SIP Parameters and slightly further down RTP parameters there are four fairly critical values. By default they'll probably be set as follows. You don't need to change them, but do make a note of them;
RTP Parameters Section
RTP Port Min:
RTP Port Max:
The importance of these parameters is great. They are the ethernet ports (virtual port numbers) on which the unit communicates by RTP (Real-time Transport Protocol) with the voice data. These numbers are important if you are using the SPA3102 inside a NAT Router / Firewall because the firewall will potentially block incoming and perhaps outgoing transactions on these ports unless instructed otherwise.
At this point, if you're operating inside such an existing router, instruct it that there should be a window in the firewall for this range. The router will probably already allow outgoing connections. Incoming connections on these port windows should point at the IP address of the SPA3102 WAN port. This IP address is the Static IP we sepcified for the unit at the beginning.
N.B. I originally suggested that the SIP port range should be open, in the belief that the SIP protocol would require inbound connections. I had a visitation recently from "Mr. Mysterious" from Hungary, who seemed able to reset my VoIP box by nefarious activity on the SIP ports. (You would think people have better things to do with their time). Not happy with this, I disabled the inbound SIP port window, and it doesn't seem to have stopped things working.
Finally on this configuration page, if you are operating inside an existing router you'll need to specify the IP address of the router, as seen by the internet. This is because the SPA3102 doesn't have a way to find this out for it's-self. When your callee sends voice data back to you, it needs to know where to send it. Behind NAT, the address of the SPA3102 is invalid on the internet. The callee must send the voice data to the router, which can then forward it to the SPA3102, as configured in the firewall above.
This IP address is critical, and is why you probably can't operate behind the firewall if you use DHCP with your ISP, and not Static IP. By nature DHCP does not garantee that your router always has the same IP address on the internet. As a consequence, if you operate DHCP with your ISP your SPA3102 can never be sure what IP address to tell the callee to send voice data back to. If you need a static IP address on the internet just contact your normal ISP. Most will be able to provide a static IP address at minimal extra cost.
Setting this parameter requires just two changes. Under the NAT Support Parameters section at the bottom of the page are the important configuration options;
NAT Support Parameters
Substitute VIA Addr:
This should be the Static IP address of your router on the internet.