BT SIP Trunk via BT 100mb Leased Line


We are installing a FreePBX together with a Sangoma SBC and the plan is to connect this to a BT SIP trunk provided via an existing 100Mb BT Leased Line (ie.synchronous ethernet connection, not ADSL).

We are prepping the SBC and FreePBX, but what we don’t know is how BT will physically present their SIP trunk to us.

Has anyone done this before? Do BT overlay the SIP trunk on the existing internet connection so it routes through our existing firewall? Or do they provide a separate physical Ethernet port from their router that we connect up directly to our SBC?

We’d like to get as much configured before BT arrive to do their stuff, but this makes a difference to the overall topology we will use.

If anyone has trodden this path before it would be good to know any gotchas too.

Have you told BT you don’t have a BT maintained phone system?

Because I used to have BT SIP over a BT Net connection the same as yours, I asked them for the SIP details and they said I can’t have them.

They said their engineer has to input the settings, I said it is my phone system just give me the details.

They said “you don’t have a BT authorised phone system, you can’t connect to our SIP channels”.
Spent weeks arguing, they ended up sending me a SIP gateway which basically converted the SIP to ISDN! BT engineer came out and configured it with BT SIP details and that was that. I was mighty annoyed with them and really annoyed I had to have this stupid SIP Gateway that converted it to ISDN and my phone system still had to work with ISDN.

So my advice is contact BT ASAP and confirm that it is going to work because there should be no reason to send an engineer, no on-site configuration is needed you just connect to SIP over the existing BT NET.
If not get out of BT SIP and find a decent provider.

I suspect the engineer is going to arrive and expect to find a BT authorised phone system.


Wow. Thanks for that BigJoe. We have told BT we are using our own VoIP phone system, but you could sense they were scratching their heads and inwardly thinking “oh”. And we are also constantly on their backs trying to get a date for the SIP trunk to be provided.

At one point they said an engineer would come and install their “box of tricks” which made me wonder if they were going to use a separate router or something for the SIP trunk, which meant I’d plug that directly into my SBC. But if they just do provision through the existing set up, as you suggest, then I’ll need to do some magic on our router to separate out the SIP trunk in that device, probably using a DMZ as the Sangoma videos suggest.

So far, I’m all configured my side, and I just want BT to do their thing so I can test and see if I cant get this rats nest working.

Why, oh why, did I put my self in a position (again) of relying on BT …

By the way the BT SIP trunk was excellent and great value, I can’t ever recall having a problem with it.

And I would still be with them if they weren’t going to force me to use the SIP gateway.

Would be interested to hear how they do end up presenting it to you though.

So we have our SIP trunk install booked for Friday. And the latest email I have from BT shows they are fully aware we have 3rd party VoIP pbx as they have stated the VoIP maintainer/supplier needs to be on site as well. They have also delivered 2 more network devices (I’ve not seen them myself so cant tell what they are), so it all looks promising.

I’ll let you know more after Friday!

Well, a complete and utter fail from BT. They arrived with the same SIP gateway you encountered, and expected to connect me up to their ISDN ports on the gateway. I said no, we are 100% VoIP and want a proper SIP trunk. So they said no problem, We’ll put our gateway on your internal LAN, and use it like an SBC and route the SIP traffic to your VoIP PBX which is also on your internal LAN. All you have to do is open up the SIP ports on your firewall.

Well, If they think I’m opening up 5060 and all the other RTP ports on my LAN so all my other LAN devices are exposed then they are mistaken! They’ve now gone away to think how they can do it properly.

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Slightly confused by your statement, you SIP/RTP ports on FreePBX must but open to the local LAN anyway for you SIP devices to work? So the BT Gateway takes external traffic and then only sends it to the internal PBX on the LAN side. That’s how an SBC works, but there is no reason why you can’t just open the FreePBX to the BT trunk.


The BT gateway has one Ethernet port. Which is on the LAN side, on the same subnet as FreePBX and it handles the incoming SIP traffic form the outside world, as well as the connection to FreePBX. And we open up our Router to allow SIP/RTP through to the BT gateway on our LAN. Do you think that’s a good (secure) topology? (It’s not how Sangoma have suggested it should be done.) I’m no expert, but I do like to be able to control the devices on the private LAN, and at least make sure they are patched and up to date. This just feels like I’m opening up my kimono to the milkman.

Well if it only has a single Eth port then it’s not really acting as a Gateway. You would be better off just allowing SIP/RTP traffics through from the BT IPs for their trunk either at a router or FreePBX firewall level.

Personally I have run loads of FreePBX systems hosted in a Cloud infrastructure with just the FreePBX responsive firewall for years without an issue.

Exactly. But BT will not tell you (as BigJoe pointed out above) what their SIP trunk IP address(es) are nor the parameters for us to register.

So, I’ve dumped BT and have tried a sipgate sip trunk, which worked fine connecting direct to the FreePBX. But now I’m trying to get it to connect through the Sangoma SBC so I have a bit of security, but for some reason the SBC is returning a 503 (Service unavailable) to the Freepbx when its trying to make a call out… I can’t seem to see from the wireshark why its rejecting the call though. It must be something in the SBC, as its not even trying the SIP trunk on the public side.

The SBC is registered happily with the sipgate SIP trunk, and I can confirm this from the sipgate side, but I just can’t seem to place calls. (Haven’t tried inbound yet, as I’m waiting for the inbound numbers to be set up by sipgate.)

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