How commmon is International blockage?

Technological Musings says:

British Telecom makes sure that no other VoIP service can work on their network by blocking commonly used ports.

Is this true? I know that Korea Telecom does this, but I’d never heard this before. The blog contains a number of errors, such as this one: “There are a lot of ISPs in the US, who make sure that Vonage doesnt work on their network,” so I don’t want to take this unsourced claim at face value.