The good news is that according to polling by Jeff Pulver, BT does not appear to be generally blocking port 5060 within their network.
However, I do not feel obliged to post a full retraction and clear BT fully, at least not yet. Here’s why. We still don’t know that the router is indeed the issue (although things are pointing that way). The customer has still not been able to get SIP to work (unless the server runs on a port other than 5060) and BT has not been helpful in any way to resolve the issue. If it turns out to be the router, that router was supplied by BT and came configured to explicitly block traffic from SIP servers and appears to be doing so in a surreptitious manner. This is possibly just a mistake, but it is still a BT issue. It appears to be a secret setting on the BT supplied router that the user does not have access to. So to imply this is a customer mis-configuration, as some have done, is a misrepresentation.
Also, it’s clear a lot of people are confused with the actual technical issues. What’s really strange about this effect is that packets sent from port 5060 are what are blocked, not packets sent to port 5060, as a lot of people have reported or misquoted. I suspect a lot of people don’t know the difference, but it is a particularly peculiar behavior not typical of consumer firewalls/NAT, that more comonly block based on the destination or target of the packets. The BT system is blocking traffic inbound, if that traffic comes from port 5060, regardless of destination port. Inbound traffic sent to port 5060 goes through fine, as long as it comes from some port other than 5060. This is clearly intended to block traffic from SIP servers, not to protect the customer’s network from malicious traffic (since it’s easy to attack the customer systems through the BT router on whatever port by sending from a port other than 5060).
UPDATE: It’s confirmed. 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.