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:
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.