The answer to the question depends on what percentage of customers have delisted their numbers and what percentage of customers have discontinued their service since the data for the phonebook's last edition was compiled.
The correct answer has already been given and is "0." Since they are in the phonebook, they are listed. It doesn't matter what they do in the future or what they have done after the book was published. The fact that they are in the book means that, at some point in time, they were listed.
Yes, I saw this answer posted. However, since the problem was not carefully worded, it does not realy have a solution. You are talking about listed people and it is irrelevant. The problem is about people who have unlisted phone numbers. You assume that "to have unlisted phone number" is the same as "not be listed in the phone book" while it is not the same.
I was able to solve Einstein's puzzle in about 15 seconds. Just go to www.google.com and type in "Einstein puzzle solution". The first page you'll see come up is this one: http://rakkav.tripod.com/homeworlds/brainstem/pages/einsteinsol.htm and there's your solution.