As usual maintain a straight face when someone tells you their number is 0207 blah, blah, blah, or 0208 blah, blah, blah, or even 0203 blah, blah.
The phone number as an IQ test, it's infallible in my experience. If people can't even get their own phone number right, can you trust them to do anything correctly?
A shop down the road still has 01 xxx xxxx above the door, a number scheme that was sent to /dev/null back in 1990, of course you can extrapolate the likely number for the shop from its location (outer London). But, I have my suspicions that the sign is intentionally wrong as a form of encoding to discourage stupid customers from phoning.