I found this issue is caused by TalkTalk Scamblock service. You have to phone them to get it turned off. If you go into your TalkTalk myaccount/myservices/homesafe settings and try to turn off Scam Protection it gives what is probably the standard customer services number. You then spend an age fighting the annoying automated robot and eventually get to somebody who will with any luck eventually turn it off. The bloke I ended up with took repeated attempts to do so before it showed as off my end and teamviewer worked. TT mention it's done via their DNS so I changed to Google public DNS on the router it case it reappears some time later as there are mentiond of this going back to 2016 which say TT removed the block.
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