Custom field unique

[bbaiatbun1]

Hello,
I've been created a custom field "Phone number" and created a form element of "list box" where i put all the phone numbers from our company. I added the custom field to a fieldset, all good ; i add a new asset, and select a particular phone number and if i add a new asses the same number added in the initial setup is availlable to add again. I want that one added that Phone number to be unique to one asset and cannot be assigned to another assset, just like an serial number on an asset that is unique and cannot be assigned to another asset.
I am using SnipeIT Version v5.3.0 - build 6470 (master)
thx