extracting phone number using api

Your question is can be answered from this reference:

https://community.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
You need an Address Book and you can retrieve the phone numbers associated to the contacts, for which you might need : Get Address Book ( API Endpoint )