Are you asking whether the RingCentral mobile app displays the caller's information when a call is answered at the office desk phone, or do you mean something else? Let us know so we can provide the best answer.

I’m asking the first part of your response 

Thank you for clarifying. Indeed, all call endpoints will display the same caller ID when answered. The caller ID shown on the mobile app should match the caller ID of the incoming call as displayed on a desk phone when answered.

That is not happening.

When my secretary answers the phone all the RC App says is unknown callers and no display of the phone number but on my desktop the phone number is displayed.

Let us assist you in investigating this matter further with the help of our support team. Please send me a direct message with your RingCentral number or the email address associated with your account.

as I said earlier, No.