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David Beckemeyer
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
If this is true, this is VERY bad behavior. 5060 is the port that SIP uses. I can understand why a phone company wouldn't want "free phone calls over the Internet" running on their system, but this is exactly the kind of behavior that makes Internet folks dislike telephone company control.

Can anyone else corroborate this fact?

VoIP stands for "Voice over IP" and SIP is the open standard "Session Initiation Protocol" used to set up calls over the Internet

UPDATE: Looks like it is a customer router issue, but still may be BT driven. Update on Mr. Blog.


They are spitting against the wind.

I was just using Skype (between UK and Japan) and I am a BT Broadband user.

Skype is not using SIP. They have a proprietary protocol.

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’m amazed every time i do it here :]

anyway most brits don’t spend too much effort exploring other cultures so it’s not big loss.

ask cisco to send a new image that removes the ACL capability.

Isn't cisco betting on VOIP?

This news item from the BBC gives some interesting context to this.

SIP Functionality: What port numbers should I use for RTP sessions set up with SIP?
You will need an even/odd port pair. The even port is for RTP, and the odd is for RTCP. Never use ports below 1024. It is strongly recommended that you avoid ports between 1024 and 49151, since these are assigned by IANA, and might be in use by some other protocol. It is highly recommended that you choose ports between 49152 and 65534. The general algorithm for this is to choose a pair in this range, and request it. If one or both are in use, try another pair until you obtain a free one.〈=en

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I've been working for the last few months with a client with a VoIP product and, out of general enthusiasm for it, I offered to help test it here in the UK using our BT Broadband ADSL service. Our broadband is a consumer, home service, and usually work... Read More