Okay, here's the lowdown on the VOIP RJ-11 jacks you'll find on many a router these days - including the ones for FTTH use. Essentially, they're an integrated version of the ATA, or Analog Telephone Adapter.

What does an ATA do? Basically, the idea is that with VOIP, instead of using the copper telephone wire to carry voice signals, the sound recorded by your microphone is converted into a digital form (the Voice) and transmitted via the Internet (Over Internet Protocol). At the other end, the digital waveform is converted back into a voice signal and carried via PSTN (the telephone network). If the person on the other side is also using VOIP, then it will be converted again into a digital format until it reaches their computer, where it will undergo the final conversion back to a voice signal and piped out through their speakers. At this point, you may have noticed that a telephone handset has both a speaker and a microphone, and is, after all, meant for telephone calls. So instead of using software on your computer to do the conversion, and then you need a mic and speakers, why not just use back your existing phones? An ATA is basically a device that allows you to plug your phone in one end and an Ethernet cable to your router in the other end. It does the conversion from a phone signal to a digital form and sends it out to your VOIP provider via the Internet, and vice versa.

Think of an ATA as the exact opposite of the old 56k dialup modems. A modem converts the digital data from your computer into voice data, which it then sends across the normal telephone network to the dialup server, where another modem converts it back into digital data. The ATA takes voice data, turns it into digital data, and sends it across the Internet to the VOIP server, where another ATA converts it back into voice data.

That's it, really. In practice, with this kind of router, which has built-in ATA functions, what you need is to sign up with a VOIP provider. They may give you an actual phone number or not, but they will definitely give you a SIP username and password, as well as various config options (SIP & STUN server, codecs supported etc.) - you plug these into your router's VOIP settings. Then you plug in a phone and you're done. The point is to make it seem as if it really is a normal fixed line service.

In your case, it seems that the Billion router has a *third* RJ-11 jack for an *actual* fixed line service, according to the writeup. The idea is that you plug in your TM phone line into this jack, so that in the event your Internet goes down, you can still call out and receive normal telephone calls on the same phones you're using for VOIP. Assuming you still have a fixed line service, of course.