I want to switch numbers on an existing desk top phone. Would like to use an existing direct number.

make sure the "existing RC direct number" is in inventory, and not assigned to anything.
Go to Phone System > Phones & Devices > User Phone
Click on the "desk top phone with an MVP license"
Click on edit number and select the correct one from Inventory. (If you have many numbers in inventory, you might need to search with the spaces in the number. For example
0123 456 789 search like this
because 0123456789 might not find the number