title: Require unique phone number in Asp.Net Core Identity
body: <p>In Asp.Net Core Identity framework, I can easily require a unique email address by setting RequireUniqueEmail = true. </p>
  
  <p>Is there any way to do the same for the user's phone number? Note that I don't want to require a confirmed phone number to sign in. The user is not required to enter a phone number but if they do, it must be unique.</p>