Welcome to the RingCentral Community

https://developer.ringcentral.com/api-docs/latest/index.html#!#RefAddressBookContacts.html

You need to retrieve the Phone Numbers on the extensions. Check the following reference: https://developers.ringcentral.com/api-reference/

For permission, submit a support ticket with the developer support team.

Check similar answer: https://forums.developers.ringcentral.com/questions/622/how-to-get-phone-number-from-contact-name-using-ap.html

To retrieve the Phone Numbers on the extensions you could use :

Get Extension Phone Number List

If you have an Adress Book and if you would like to use it to retrieve the phone numbers associated to the contacts, then you might need :

Get Address Book ( API Endpoint )

If you do not have the permission to see this endpoint, please open a support ticket with the developer support team.