I've just sent a test email to firstname.lastname@example.org without a problem.
If you think BT or any other ISP can totally eliminate spam and phishing emails, I'm afraid you are deluding yourself.
As the original poster, I long gave up on pursuing this issue. The BT Phishing reporting process is completely unreliable, sometimes it works, sometimes it doesn’t.
I now just forward any problem e-mails to email@example.com which works every time and then delete the offending message. I’m not sure whether it’s just coincidence but since I started doing this there has been a very significant reduction in the number of phishing e-mails I receive: it’s down to about one or two a month from upwards of two or three a day.
Just had the same rejection from "firstname.lastname@example.org" this morning, both by forwarding the suspect email and by attaching it to a new email, so this problem is still on-going on 29th July.
I did not say anything about BT or any other ISP being able to totally eliminate spam e-mails, so nobody is deluded about anything. It would just be nice that something that BT opted to put in place and actively encourages people to use worked reliably all the time.