What is the best way to add new line of service and order a new desk phone?

I have added 3 new users, phones, digital lines. I ordered the phones separately from the user creation. It made if difficult to complete and assign phones. One support tech had me change the newly created user to use a soft phone app. The next tech when i had to do the 2nd one a day later had me unassign the recently created number.. I did this backwards and I'm looking for the proper documentation to adding new line of service with a desk phone.