Re the illegal scammers and spammers, both inside and beyond EU...
If they are simoly crooks, then obviously won’t take any notice of GDPR.
But even crooks must still find a source of data for their illegal activities. Often, their source comes from legitimate databases that have leaked. If GDPR reduces the number of databases on which I appear, I reason it will also reduce the probability of leaked data that includes my details. That should in turn reduce the number of scammers and spammers who get hold of my contact details.
Question... will BT have to ask customers whether they wish to “opt in” to directory listings? Obviously, people are already opted in unless ex-directory, but I don’t think companies can rely on old agreements, unless these old agreements already met GDPR’s stringent standards. Specifically, you should now have to explicitly “opt in”, rather than “opt out”, to use of your data. I never saw or signed the contract all these years ago (other half sorted it out) but it strikes be the BT phone book listing was perhaps an “opt out” model.