I think I have found the API endpoint I am looking for

https://developers.ringcentral.com/api-reference/Phone-Numbers/listExtensionPhoneNumbers

but this doesnt give me the result set I expect, even when I add the filter below, I still get multiple records back for just one extension ID. Any help anyone could provide would be greatly appreciated?

usageType=DirectNumber

Based on your own comment, you want to read direct number of all extensions under your account, right? If that is what you expected, use the account phone number endpoint.

var endpoint = `/restapi/v1.0/account/~/phone-number` 

var params = {

        usageType: [ "DirectNumber" ]

        }

Well no what I need comes from the extensions endpoint. Unfortunately, the phone number there, labeled businessPhone, is nothing but the best contact phone number, which is blank for most of our records. I thought it was the direct RingCentral number for that extension. Since that is not the case, I posted this question, but soon after found the endpoint reference page that I listed in my comment, which pulls phone numbers for a given extension id. I then added the uri parameter to limit the results to "DirectNumber", but I am still getting multiple phone numbers for a single extensionId?