Database unique field

I am creating a table of user with fields name, email, address,phoneNumber.
I want to set the phoneNumber as a unique field but not getting any option for doing so.
Can anyone have any idea?

This post might help.

Edit: I should add that I know you’re not necessarily asking to use phone number as a username, but the point is you may have to add workflow steps to check if a phone number is already in the database and if so prevent it being added again. The post I link to above discusses a similar thing.