DeskWatcher

Yes, talktalk are also notorious for mass-blocking Teamviewer (and others) at DNS level with their router/broadband services

You could have stopped after “notorious” and been correct on all levels.
L7 detection of the application traffic is a good killer too.

Thank goodness for 1.1.1.1

Default circuitless IP on many Cisco installations. There used to be quite a few out on the internet until the net authorities started allocating the 1.0.0.0/8 block