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 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, to ensure Bob can encode the message so that Eve cannot read your phone number?
Since you are just “checking”, you ask him to call you at a certain time. If he doesn’t, he doesn’t have your number.