Phones and Phone Hardware have different functions within the platform.
After some time using the platform, you may begin to notice that the way in which Users and their Devices interact with each other is a little confusing. For example you may notice that you can create a user without a phone, add a phone in the phone hardware, but you can't link it to the user and if you try the phone won't register, Why is this you ask?
To understand these interactions it's best if we explain exactly what each of these objects are, and what they are used for.
The User - How do I configure a user? - Understanding The User
A user is basically a selection of features and switch logic that make up a singular entity, it contains things like voicemail, caller ID, and Answering Rules. However, a user on it's own is unable to make/receive calls or make any kind of SIP connections, this is where "Phones" come in.
Phones - How do I configure a new SIP device/phone?
When you open up a user, one of the tabs along the top is labelled as "Phones", to explain how this works, it might be easier to instead think of this tab as SIP Accounts, each phone under the phones tab is an individual SIP Account that is associated with the user it's under. You can then use these SIP Accounts to register a soft phone, or some physical phone hardware.
Phone Hardware - How do I configure phone hardware in the domain?
If you navigate to your inventory, and select the "Phone Hardware" tab, you are greeted with a list of all the physical SIP devices you have configured on your platform, these devices contain information like the MAC address and Configuration File. Adding a phone to the phone hardware tab creates an entry on our provisioning server for said device. However, in order for the device to register it needs a SIP Account on one of the lines of the device.
So now you understand we can begin to answer the question we asked at the start "You can create a user without a phone, add a phone in the phone hardware, but you can't link it to the user and if you try the phone won't register, Why is this?"
The reason the phone hardware won't register is because there's no configured SIP account for it to register to, and the reason you can't link it to the user is because you aren't linking the hardware to the user in the first place, you're linking it to a Phone configured inside the user which contains the SIP Credentials needed for registration.