The following Users API https://developer.zendesk.com/api-reference/sales-crm/resources/users/ has a phone_number field which is always null. However Sell does store the given User's phone number under the "Voice and Text" setting. It would be very helpful and accurate if this phone number was mapped correctly to the API.
Attached a screen shot of which Sell phone number I'm referring to.
Right now we have to manually do a workaround by having our users copy and paste their Sell phone number into a custom app, for us to be able to query these numbers through the API