Adding a 3rd party phone as a physical extension that rings the primary number

This is what I understood:
You have a user extension, which by default has the RingCentral app assigned to it. 
You want to assign a 3rd party phone to it.

How to do it:
Phone System > Phones & devices > User phones > click on the extension > Change phone

In the pop up that opens select Bring your own device. The pop up will ask you to select the device model. If you are able to select it - this means it’s one of the supported phones  and you’ll be able to set it up via assisted provisioning. If you are not able to select the device model, this means it is not a supported phone and you have to do manual provisioning - click on Set up manually using SIP(pic below)
 

After you do this the phone will ring to the direct number of the user extension it is assigned to. 
If you want the company main number to go to the phone, then you need to set the company main number to route calls to that user extension, like this:

Phone System > Auto-Receptionist > General settings > Call Handling > select “A specific user” and then select extension

You can think of Auto-Receptionist > general settings as the place where you configure the call handling for the company main number.

It is confusing to have individual user accounts identified as extensions. This is one user, me, that wants two phones to both ring when someone calls the one number. One of those phones its in my office and one phone is in my home. If I am in my home and the phone rings, I can answer the phone. If I am in the office I can answer the phone

​@Zarthan 
If you want to have 2 phones on a single user, then that user needs to have 2 licenses. look at the 2 pics below.

Go to Users > click on the user extension > Devices & numbers > check how many licenses you have on that user. 
If you need an additional license you can purchase one from the “Add Phone” button

 

should have mentioned that you can save on purchasing an additional license if you use just a single deskphone together with one of the ringcentral apps (desktop app, web app or mobile app) instead of using 2 deskphones.

I do have an additional license. In this situation I am going to be using an ATA and connecting cordless phones so I can have them in convenient places around the house. I also have Vonage and it was simple to add the second phone/ATA and add the cordless phones. Ring has made it far more difficult and expensive than it needed to be