Require unique phone number in Asp.Net Core Identity

The easiest way might be to simply search for the phone number in your controller...

bool IsPhoneAlreadyRegistered = _userManager.Users.Any(item => item.PhoneNumber == model.PhoneNumber);

You can try this, basically enforce it at the Db level first, followed by implementing proper checks at the Manager level.

At DbContext, I declared the indexing and uniqueness of both the username and email properties.

// ================== Customizing IdentityCore Tables ================== //
        // https://stackoverflow.com/questions/30114751/renaming-identity-tables-with-ef6-migrations-failing

        builder.Entity<User>().ToTable("Users").Property(p => p.Id).HasColumnName("Id").ValueGeneratedOnAdd();
        builder.Entity<User>(entity =>
        {
            entity.HasIndex(u => u.UserName).IsUnique();
            entity.HasIndex(u => u.NormalizedUserName).IsUnique();
            entity.HasIndex(u => u.Email).IsUnique();
            entity.HasIndex(u => u.NormalizedEmail).IsUnique();

            entity.Property(u => u.Rating).HasDefaultValue(0).IsRequired();
            entity.HasMany(u => u.UserRoles).WithOne(ur => ur.User)
                .HasForeignKey(ur => ur.UserId).OnDelete(DeleteBehavior.Restrict);
            entity.HasMany(u => u.UserClaims).WithOne(uc => uc.User)
                .HasForeignKey(uc => uc.UserId).OnDelete(DeleteBehavior.Restrict);
        });

And for the Manager level code:

/// <summary>
        /// Sets the <paramref name="email"/> address for a <paramref name="user"/>.
        /// </summary>
        /// <param name="user">The user whose email should be set.</param>
        /// <param name="email">The email to set.</param>
        /// <returns>
        /// The <see cref="Task"/> that represents the asynchronous operation, containing the <see cref="IdentityResult"/>
        /// of the operation.
        /// </returns>
        public override async Task<IdentityResult> SetEmailAsync(User user, string email)
        {
            var dupeUser = await FindByEmailAsync(email);

            if (dupeUser != null)
            {
                return IdentityResult.Failed(new IdentityError() {
                    Code = "DuplicateEmailException", // Wrong practice, lets set some beautiful code values in the future
                    Description = "An existing user with the new email already exists."
                });
            }

            // Perform dupe checks

            // Code that runs in SetEmailAsync
            // Adapted from: aspnet/Identity/blob/dev/src/Core/UserManager.cs
            //
            // ThrowIfDisposed();
            // var store = GetEmailStore();
            // if (user == null)
            // {
            //     throw new ArgumentNullException(nameof(user));
            // }

            // await store.SetEmailAsync(user, email, CancellationToken);
            // await store.SetEmailConfirmedAsync(user, false, CancellationToken);
            // await UpdateSecurityStampInternal(user);

            //return await UpdateUserAsync(user);

            return await base.SetEmailAsync(user, email);
        }

This way, we retain .NET Core Identity Code's Integrity while enforcing the uniqueness of the property/properties that we want.

Note that the samples above are for email as of now. Simply do the same and then instead of modifying SetEmailAsync, work on SetPhoneNumberAsync at UserManager.cs.