I was thinking of testing that 30% thing last night, as it happens. Many years ago I set up the membership database for a London-based organisation, which meant I was the "keeper of the phone numbers". Last night - via something else which prompted the thought - I was debating calling all the numbers I have on file to see which (indeed, if *any*) are still extant and the same person.

Although I havn't moved my phone number has changed; I doubt that more that 3-5% might still be accurate ...