Question: You need to check that your friend, Bob, has your correct phone number, but you cannot ask him directly. You must write the question on a card which and give it to Eve who will take the card to Bob and return the answer to you. What must you write on the card, besides the question, to ensure Bob can encode the message so that Eve cannot read your phone number?


Solution: As the problem is only to check that Bob has phone number or not.
So You will write "Give me call Bob" If he calls you back you know he has number otherwise not.