Looking up users in AAD B2C using extension attributes or unusual standard attributes

This is a follow-up to this question. I need to query Azure AD B2C to lookup a user using an employeeID which, for reasons related to what fields our account provisioning solution can deal with, is currently stored in the telephoneNumber field.

I have created an Azure Active Directory TechnicalProfile which specifies the telephone number as the InputClaim:

 <TechnicalProfile Id="AAD-UserReadUsingEmployeeId">
      <Metadata>
        <Item Key="Operation">Read</Item>
        <Item Key="RaiseErrorIfClaimsPrincipalDoesNotExist">true</Item>
        <Item Key="UserMessageIfClaimsPrincipalDoesNotExist">An account could not be found for the provided ID.</Item>
      </Metadata>
      <IncludeInSso>false</IncludeInSso>
      <InputClaims>
        <InputClaim ClaimTypeReferenceId="telephoneNumber" PartnerClaimType="employeeId" Required="true" />
      </InputClaims>
      <OutputClaims>
        <OutputClaim ClaimTypeReferenceId="objectId" />
        <OutputClaim ClaimTypeReferenceId="userPrincipalName" />
        <OutputClaim ClaimTypeReferenceId="otherMails" />
        <OutputClaim ClaimTypeReferenceId="signInNames.emailAddress" />
        <OutputClaim ClaimTypeReferenceId="signInNames.phoneNumber" />
        <OutputClaim ClaimTypeReferenceId="displayName" />
        <OutputClaim ClaimTypeReferenceId="givenName" />
        <OutputClaim ClaimTypeReferenceId="surname" />
        <OutputClaim ClaimTypeReferenceId="telephoneNumber" />
      </OutputClaims>
      <IncludeTechnicalProfile ReferenceId="AAD-Common" />
    </TechnicalProfile>

However, when compiling this policy I get the error:

Input Claim 'telephoneNumber' is not supported in Azure Active Directory Provider technical profile 'AAD-UserReadUsingEmployeeId'

I understand from the documentation here that telephoneNumber can only be used as persistentClaim or OutputClaim... so I suppose I may be hitting that limitation. I thought I could wiggle out of this by querying Azure AD B2C using an extension attribute instead, but there I get confused:

• The answers to the question I linked to earlier state that you can't query Azure AD B2C through some random attribute, because the attribute needs to be unique
• The FIDO2 sample policy here does use an extension attribute to query Azure AD B2C and lookup a user, so it would appear to be possible in some conditions.

So, would I be correct in understanding that

• You can use any extension attribute to lookup a user in Azure AD B2C, and it's on you to ensure that the attribute values are unique
• You can't use most of the standard attributes (such as telephoneNumber) to look up users

Or is there something obvious I'm overlooking in the way I've configured the TechnicalProfile?

These are correct:

• You can use any extension attribute to lookup a user in AAD B2C, and it's on you to ensure that the attribute values are unique
• You can't use most of the standard attributes (such as telephoneNumber) to look up users