Teams Voice with Direct Routing and NonUniqueOnPremLineURI

I've been bashing my head for a week+ now on this.  With a customer (already opened an MS ticket, but they are running in circles), assigning phone numbers to users using `msRTCSIP-Line` field on-premises.  When the accounts sync, the numbers are getting rejected with NonUniqueOnPremLineURI and the full text:

<ErrorRecord>
<ErrorCode>NonUniqueOnPremLineURI</ErrorCode>
<ErrorDescription>The value of the MSRTCSIP-Line URI field in your local Active
Directory conflicts with another user. Lync Online features will work except for
Lync-to-phone calls. To resolve the conflict, correct the value in your local Active
Directory. After you correct the value, the value will be updated in your Office 365
directory during the next Active Directory synchronization. If the error persists,
please contact Microsoft support for assistance.</ErrorDescription>
</ErrorRecord>

If I do the following in an MS Teams PowerShell prompt:

Get-CsOnlineUser -Filter "onpremlineuri -eq 'tel:+11234567890'"

A single entry, the one I assigned the value to, is returned.

If I do the following in AD:

get-adobject -Properties userprincipalname,'msrtcsip-line' -LDAPFilter '(msrtcsip-line=tel:+11234567890)'

A single entry, the one I assigned the value to, is returned.  I purposely used `Get-ADObject` just in case something assigned it to a computer, or something stupid (not possible because that attribute only exists on users).

Anybody seen sync misbehave like this? I've even moved users outside the scope of sync to "clear" MS Teams, but still run into the same issues when moved back.