Office 365 – The phone number you entered has already been registered by someone else.

Symptom: You receive the following error when trying to enable someone for a Unified Messaging mailbox on Office 365 (Office 365 Admin Portal -> Exchange -> User Account -> Enable Unified Messaging -> Browse for UM mailbox policy).

error
The phone number you entered has already been registered by someone else.
The phone number you entered has already been registered by someone else

Resolution: This was caused by having a duplicate UM Voicemail box number.  You can run the following powershell commands to identify which user has the duplicate number assigned to them.

Set-ExecutionPolicy unrestricted
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $session
Get-Recipient -ResultSize Unlimited | where{$_.emailaddresses -like “*EUM:*PHONENUMBER*”} | fl displayname,emailaddresses

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