OK, the area codes of various UK cities have changed over the past decade or so, especially in London, and usually with an extra digit or digits added to the 'phone number. Some confusion is inevitable. But every time it has been explained clearly what's the code and what's the extra digit. (I.e. new code for London - 020, add 7 or 8 to the old number). And every time, without fail, people have tacked the extra digit onto the code, i.e 0207 or 0208. And then they try to call Barnet from Croydon without the 8 and wonder why they can't get through.
It has to be said, though, that British Telecom don't help themselves by printing both the new area code and the extra digits in the area codes section of the 'phone book, thus reinforcing the error.