BT appears to be blocking third-party VoIP

I’ve been biting my tongue on this since I first ran across it several months back. But now I have to say something. If someone can prove me wrong on this, fine, I’ll post a retraction, but now I’m going to say it: British Telecom appears to be explicitly blocking VoIP for their DSL subscribers.

I’ve worked with an associate to examine the situation and all signs point to an explicit blocking of VoIP. In Cisco ACL-speak, it appears there is a rule somewhere in the BT network being applied to inbound packets of the form:

deny udp any eq 5060 any

What this says is “block traffic coming from external hosts to my DSL customer if it comes from port 5060.” It turns out this is exactly the type of packets that would come from a SIP server. It appears these packets never make it from the SIP server to the customer at all, so this is not an issue of the customer-side firewall.

It does not appear to be a general issue with UDP or VoIP packets either. If a server is configured to use a different port (not the standard SIP port of 5060), it all works fine, even using the exact same protocols.

What amazes me about this is how blatent it is. BT is actively blocking just this one specific type of traffic. BT obviously knows port 5060 is the SIP port and they know this will prevent their customers from using many (most?) VoIP services. This means this is an intentional and willful act. And they are not even trying to hide it. They could be smart and just drop a packet here and there or slow some down to mess with VoIP. But no. They brazenly block it, flat out. In our faces. Take that. What are you going to do about it?

In the US, I think we call it restraint of trade, but I have no idea if such things apply in the UK.

UPDATE: BT is not be blocking at the network. Yeah! The BT Voyager 2000 router was at fault. Getting a different code release installed on the BT router permits port 5060/SIP traffic.

29 comments for “BT appears to be blocking third-party VoIP

  1. Just a clarification – this presumably only applies to BT retail DSL customers. There’s a choice of 20+ other ISPs using BT as a wholesale provider. And you wonder why BT are bleeding customers…

  2. why i’m not astonished at all. BT are not mad to avoid the ‘profitable ethics’ of the english business culture as they want to survive. it’s like if one had any choice to find better provider in uk?
    i’m sure the easiest way to find whether blocking voip is true is to read their ‘small letter’ terms and conditions. i’ve never spent so much time reading terms and condition in any other country and I

  3. I work for a SIP Telephony provider in the UK. We found that a few BT customers had problems like this, and eventually I put it down to being either one of two things:

    1. BT were blocking port 5060, or
    2. The BT supplied ADSL router was

    It turned out to be (2).

    The Thompson/Alcatel Speedtouch 510/530 router has a SIP ALG (Application Layer Gateway) that intercepts packets on port 5060, possibly with the best intentions, but ends up mangling them.

    After a few weeks of just having blacklisted the Speedtouch routers though, we eventually discovered a solution.

    The solution is simply to telnet to the ADSL modem and remove this binding:

    telnet [or replace with the correct ip address]

    Connected to speedtouch
    Escape character is ‘^]’.
    Username : root
    Password : ********
    … welcome message trimmed

    Unbind the SIP protocol and reboot the modem:
    =>nat unbind application=SIP port=5060
    =>config save
    =>system reboot

    To check whether the SIP protocol is bound in your modem use this command to show the list of bindings (only showing SIP here):

    =>nat bindlist
    Application Proto Port
    SIP udp 5060

    If that line doesn’t appear, everything should magically start working again.

    Naturally this doesn’t apply to other routers, but we now have one BT Broadband customer who is happily using SIP, on port 5060, with our service.

    I have to admit that before we actually realised it was the router (it worked just fine with a USB modem and no NAT) I thought BT were being rather naughty.

    BT Technical Support, when contacted, apparently (i didn’t call them myself) suggested that our customer just buy a Speedtouch SIP phone, and use BT Broadband voice, as that would work fine. Very cheeky of them.


  5. Thanks everybody.

    It is the bt 2000 voyager router. not a Thompson/Alcatel Speedtouch.

    So BT, at what release of the Voyager 2000 code did they unblock port 5060? Looking at the router settings, there is no mention of SIP or VoIP or port 5060 at all, and no indication that it is blocking port 5060. What other ports are blocked in this surreptitious manner and how is a customer supposed to know if there is nothing in the router settings?

  6. It would be REALLY useful to get an ‘official’ BT line on this matter.

    The line from ‘anonymous’ sounds very feasible and fits with my personal experience. We supplied a trial Gossiptel service to Azeem Azhar, a journo doing a comparison between different VoIP offerings back in July this year. He encountered all kinds of problems getting the service running – resolved by replacing the BT router that BT had thoughtfully supplied as part of their BT Broadband voice (MGCP) offering!

    I would think it highly unlikely thay BT WOULD implement a measure as blatent as port blocking – much easier to implement a MGCP friendly/SIP blocking measure on DSL routers!

  7. It’s quite likely that the Voyager Router also has an ALG in it that is intercepting the SIP packets and then trying to do something clever with them.

    Bear in mind BT itself doesn’t actually make most of the products it sells under it’s own name.

    I would view this as a glitch in the BT Voyager router that needed to be fixed rather than BT intentionally blocking the service.

    Either way, it comes down to either one of two things;

    1. BT were ignorant to the flaw in their hardware, or
    2. BT blocked it intentioanlly to strengthen their monopoly.

    I have come to believe (1) but the more cynical amongst us won’t have any trouble in beleiving (2)

  8. Change the port? Not so big a deal, tell your service provider of the SIP service. 5060, that’s just your default. Cheers.

    Ryan Gibson,

  9. Hi Ryan, that’s the problem with the bug in the BT router. It’s the server that has to change the port. The user cannnot change the port the server uses. Changing the port the client/device uses doesn’t fix it. The router was blocking based on where the packets were coming from not where they were going to.

    Ultimately, the fix was to upgrade to other code on the BT router that didn’t have this problem.

  10. I am with ananymous on this one …. my sip softphone works fine with BT Openworls on the USB modem but not with the Thompson/Alcatel Speedtouch 510/530 router . Unfortunatly I still cannot get it to work even after following the instructions.

  11. Just thought I’d comment that I appreciate the SpeedTouch info, I’m trying to use Vonage’s SoftPhone and was having a heck of a time. I can now get past the "Timed Out! Contact Network Admin!" and am just getting an "Awaiting proxy login information". One step closer I guess.

    Vonage support blows.

    Thanks again!

  12. hey pplz how do i BLOCK BT on our adsl modem? i have a speedtouch 530 modem and i need to be able to block it from my computer (he connects to the modem on another comp) if any1 nos how 2 that’ll be GREAT!

  13. I have just received an email from a client in the BVI’s saying that Cable & Wireless have apparently block the VoiP port and their Vonage unit does not work any more! Anyone have any info on this?

  14. Hi all,

    I have a Speedtouch 510 too – but not with BT. It appears that it’s not BT but Thompson/Alcatel causing this problem. The motivation is probably that their new router, the Speedtouch 716 contains VOIP and the older 510 probably shares some of the firmware.

    =>nat unbind application=SIP port=5060
    =>config save
    =>system reboot
    from above fixed it for me immediately with

    Thanks for your help.


  15. I work for the BT ADSL helpdesk and I can assure everyone we do not block any ports. All BT offer is a standard internet access package. Why do you think you are not issued with a password when you sign up? because it does not go through the home gateway router. Same problem with people calling asking us to unblock ports so that they can use their router’s for online gaming. Why would we supply a router for people to use for online gaming and then block the port?

  16. Hi, i am using the D-link ADSL router (DSL-G604t)
    I would like to know is there any way to block other users using BT in my network?


  17. =>nat unbind application=SIP port=5060
    =>config save
    =>system reboot

    Works also for us on an alctale 510

    Thanks for this valuable information

  18. =>nat

    =>config save
    =>system reboot

    Works also for us on an alctale 530

    Modem was supplied be Telstra Bigpond

  19. I am trying to open SIP port 5060. I have a speadstream modem with which following commands do not work. Any help to unblock the Sip ports. Thanks


    =>config save
    =>system reboot

  20. Hello,

    I use VoIP-SIP phone and it was working fine…Now I moved to a different location and it was not working..I guess my ISP has blocked 5060…Is there any way to check from my home PC. What could the problem be ???

  21. I use a dial-up is working fine…but when I use ADSL for internet connection my VoIP-SIP phone is not working…can any one give the solution for this ????

  22. Hi all,
    I am using Draytel VOIP service and until a recent upgrade to BT 8Mbs Option4 my Dratel VOIP service worked fine.. Now I get the message NAT/FIREWALL BLOCKED where before I got the message NAT/Firewall Restricted Cone NAT..
    The problem this is causing is that on occasions during a conversation the sound both in and outbound is cut.
    Does anyone know the cause of this?

    Many thanks


  23. I pull two voip lines from my B2B ATA device and they work fine on my existing broadband. When I switch to SBC DSL, I lose line 1 which is on port 5060. SBC says they have port 5060 open. Line 2 is on port 5061 and works great. What’s up ?

Comments are closed.