A Sip User Account is a endpoint where you can make and receive calls from.
When you purchase a Phone Number, your Phone Number is your endpoint where you can make and receive calls.
Any phone in the world can call your Phone Number.
You add your Sip User Account credentials (username and password) to any sip capable device to make and receive calls.
Think of a Sip User Account as a dedicated phone line or phone number, just like any traditional landline phone or mobile phone where you can make and receive calls.
The only difference between a sip phone and your traditional landline phones and mobile phones, sip works over the internet only. You only need a 3G/4G mobile or WIFI access to the internet to receive and make calls using your Sip User Account (Phone Number).