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Message 1 of 13

BT FTTP, Residential Digital Voice, and thirdparty routers.

Apologies for going over old ground, and I’m sure the answers are in these forums somewhere, but I can’t find definitive statements on my situation, which I’m sure is not uncommon. 

I wish to:

  • Order BT FTTP broadband for residential use (replacing existing BT FTTC)
  • Include digital phone keeping my existing number 
  • Use my existing (or indeed new) thirdparty networking equipment for routing and wireless etc in my home
  • Use my existing DECT phone equipment (ideally)
  • Avoid double NATing.

So my questions are: 

  • Basically, can I do this?
  • If so, how?
  • Would the use of a “BT Business Smart Hub 2” help achieve this in any way (e.g. with bridging mode, while retaining the ability to use residential DV)?

If this is not possible, I would welcome suggestions for alternatives (but still using the BT FTTP offerings), particularly with reference to how migrations can be effected.

(I have deliberately not said what my existing kit is, as I aim hoping to elicit responses that help me understand the “what” of the process(es) that need to be done, rather than some settings that work for specific hardware.  I will almost certainly be changing the internal networking kit anyway).

Thanks all!

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Message 2 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.

As has been posted countless times, not least in the DV FAQ, the BT Smart Hub 2 is the ONLY device that can supply Digital Voice and has to be the first device connected to line.


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Message 3 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.

You can connect your own 3rd party router to the SH2, while keeping DV, if you want to do that:

If the 3rd party router supports AP(Access Point) mode, just activate that and connect your 3rd party router via its LAN port to one of the LAN ports of the SH2. Then turn off the WiFi of the SH2.

However, if it doesn't or you want to use all the features of your 3rd party router:

1.) Connect your secondary router's WAN port via ethernet to one of the LAN ports of the SH2.

2.) Then make sure to turn off the DHCP, NAT, WiFi of the SH2. Give your secondary router a static IP, then put that IP in the DMZ of your SH2(To avoid any double natting issues), and you should be good to go.

3.) If you want IPV6 just make sure to enable "pass through(bridged)" mode on the IPV6 page of your secondary router.

(If you want to use your BT TV box over WiFi, make sure to get a router with IGMP snooping)

4.) Reboot them both for good measure and then everything should be working.

(Note: Even if you switch ALL wireless off on the SH2, the DECT will still function fine.)


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Message 4 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.

You will still be double natting even if you put the third party device in the DMZ of the SH2.

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Message 5 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.

Oh yep, but it hardly matters these days.

I'm able to host multiplayer games on my laptop and videocall just fine with it setup like this, without any latency loss! I think putting my secondary router in the DMZ of the SH2 helps though. It doesn't remove the problem, but it helps mitigate it.

Nothing complains about a double NAT, and on my PS5 it's still a type 2, the same as it was when I just had the SH2 hooked up. 

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Message 6 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.


You are really between a rock and a hard place if you wish to keep all your requirements.

You can mix and match, but you won't be able to have them all.

BT digital voice not only utilises DECT technology for the BT supplied DV handsets, it also supplies a built in ATA that is presented as a standard phone socket at the rear of the hub. It also supplies DV adapters which are effectively portable analogue phone sockets that connect to the hub via DECT. Your existing DECT base station can therefore simply be plugged into either to maintain your existing set up.

However, if you stay with BT DV, you will either have to utilise your third party router as an access point rather than make use of its routing capabilities or connect it as a second subnet with double natting which may or may not cause problems.

The other alternative is to use a third party VoIP provider with your third party router rather than use BT DV. The downside being that you lose the built in ATA and DV adapters of the hub, you may also have to change number.

Message 7 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.

Thank you both @licquorice and @c64z86   This is what I feared.  Since I’m keen to stay with BT for the broadband service, my two options appear to be 

  1. Live with the double NAT - that won’t be a problem, right up until it is, and then becomes a very awkward problem.
  2. Try to get my FTTC broadband migrated to FTTP via BT at exactly the same time as I get my phone number ported to a third-party VoIP provider.  What could possibly go wrong there?

I think I’ll try a combination of these:  I’ll take option 1, and once it’s all migrated/ported and if/when the double NATing becomes an issue, I’ll port the phone number a second time to another VoIP provider and then abandon the SH2.  (In this case I’ll end up with more kit to support the phones and paying for the phone access twice if I’m still in contract, but small beer in the grand scheme of things).

Thanks again for the advice.

Message 8 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.

I hope you guys don't mind me asking, but what issues can double natting potentially cause?

I read that it can affect multiplayer hosting gaming, video calling, home server sharing, port forwarding... but am I right in thinking that DMZ helps to avoid these issues?

I mean I haven't  noticed any problems so far, but what issues could I/we who have 2 routers on our networks run into, even with DMZ enabled?

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Message 9 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.


If you decide to move to BT FTTP and get DV  with a new broadband and phone contract and then subsequently decide at a later date to port number to a separate VOIP provider then that will terminate your existing broadband and phone contract.  You will then need to negotiate a new broadband only contract with BT

At present there is no system which allows you to port number to VOIP provider and maintain your existing contract

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Message 10 of 13

Re: BT FTTP, Residential Digital Voice, and thirdparty routers.


I’m not an expert in NAT but my understanding is as follows:

Firstly, there are two things here that get tied up together today.  Address translation (NAT) and port translation (PAT).  Modern NAT is a combination of both.

Basically, NAT is a complicated beast at the best of times and doing it twice just complicates things even more.  Instead of translating from the private (internal) address to the public (external) address it has to do private address of your network > private address of the network between your first and second routers > public address etc.  If everything is configured correctly it shouldn’t be a problem but more complexity means more opportunity for screw up.  Also, it introduces more latency, of course, even if it is working properly.  Somethings, like UPnP support, Xbox, multi-player games etc. don’t like it.

The port numbers also need translating/forwarding.  The DMZ host setting helps here as all ports are just forwarded from the SH2 to the DMZ host, (unless subject to a more specific rule).  Basically, it simplifies the port forwarding on the SH2 end but the actual address translation still needs to occur.